SawzAll

Ideally they would all be equal, but right now heat is the #1 element, then physical, and energy sort of needs help.  But it really depends on what you want.  Really, you can beat the entire 1v1 campaign with an Avatar-style boiler (heat mech), so it just depends on what you want.  I'd say try to play what the game gives you.  If you get a bunch of nice energy premium items, try to play energy first.
Another tip for progressing that's going to sound counter-intuitive, but it's true: do NOT food all your items.  Keep at least one of every epic-to-divine and one of every premium item.  If you can't transform something to legendary or mythical, wait.  Don't push forward and sacrifice a good item just to transform another item.
I have recently created an SM padawan clan specifically for helping new players.  Send me a message if you're interested.

Nothing is an exclusive limited time offer.
That offer is only for newer accounts.

NGL this is the first thing that came to mind.
 

You don't need nightfall.  But yes I think it's good.  add resistance drainers into the spare weight when you remove Nightfall.

You don't need nightfall.  But yes I think it's good.  add resistance drainers into the spare weight when you remove Nightfall.

I am posting this specifically to catalog good builds that have a future in top ranks, possibly.  This is basically a list of past and present META mechs but in a middle ranks form that doesn't have a lot of premiums.  This is basically a list that I believe will help middle rank players prioritize what they upgrade so that they can get to top ranks faster.
Note that I will be using monkey torsos because monkey torsos basically rule the middle ranks.  You'll stay at rank 6/7 with these builds.  This is basically the step right before you would end up getting a vest and max protector / defense matrix and ending up in rank 4-5, or whatever other premium resistance, etc.  This list assumes monkey torsos but no quad cores or overload preventors (no big deal: you can substitute for the modules below as you see fit).
These will particularly be immune to the average energy mech if you max the modules out.
Avatar-style boiler:

Mid-ranks heat damage #1:

Heat Damage #2:

Note that I believe heat point is better than heat protector drone but it may or may not work out for you.
Your average energy drainer:

Note: I believe face shocker is a bit better, but it depends on whether you can tolerate the 3 turn thing.
Claw Physical from old META:

My own "all range" physical that worked remarkably well in mid ranks (especially against energy mechs)

Obviously you can substitute another frantic if you want, or reckless plus night eagle, but this one worked very well against middle rank energy players (assuming they didn't have a krap ton of premiums)
Untested Dual Nightfall
 

I have not tested this, but I have faced this in the arena once or twice.  I can't remember who played it, but dual nightfall is genius.  Night eagle helps keep people out of frantic range, and back breaker is only used to knock people into nightfall range.  So this is a testing build.
Once you get premium resistance, you'll "graduate" out of this "middle ranks META".  (This isn't META, by the way.)  But it was an idea for people who want to upgrade stuff.  The avatar style boiler only gets better with premiums, and can beat some of the hard RAID and gold portal levels.  As well it will help you beat 1v1 campaign.
Everything else is just your "average" example.
Again, this isn't META, this is just a helpful idea post.  I have tested all of these except the dual nightfall and they work great in middle ranks.
EDIT: some caveats though, really quick:
First, I only run rounded in the arena, that's just me.  I don't play the whole game of running soft counters and then trying to use matchmaking.  This means that I had a bunch of maxed modules, so when I got MPV and some defense matrices, I literally shot up from rank 7 to rank 4.  Rounded is just how I roll.  Not saying soft counters aren't valid, it's just not me.
Second, I would intentionally run a randomizer in the arena in terms of what mechs I picked in middle ranks.  This is because I would get at least 1 win per day of someone who either picked the wrong mechs or tried to switch mechs to compensate.  I was taught if someone switches mechs, I should switch mechs too.  This means one win per day was someone quitting out because they picked the wrong mechs.
If you want to run soft counters or hard counters, this is not the list for you.  Nothing wrong with counters, I'm just not into them.

I am posting this specifically to catalog good builds that have a future in top ranks, possibly.  This is basically a list of past and present META mechs but in a middle ranks form that doesn't have a lot of premiums.  This is basically a list that I believe will help middle rank players prioritize what they upgrade so that they can get to top ranks faster.
Note that I will be using monkey torsos because monkey torsos basically rule the middle ranks.  You'll stay at rank 6/7 with these builds.  This is basically the step right before you would end up getting a vest and max protector / defense matrix and ending up in rank 4-5, or whatever other premium resistance, etc.  This list assumes monkey torsos but no quad cores or overload preventors (no big deal: you can substitute for the modules below as you see fit).
These will particularly be immune to the average energy mech if you max the modules out.
Avatar-style boiler:

Mid-ranks heat damage #1:

Heat Damage #2:

Note that I believe heat point is better than heat protector drone but it may or may not work out for you.
Your average energy drainer:

Note: I believe face shocker is a bit better, but it depends on whether you can tolerate the 3 turn thing.
Claw Physical from old META:

My own "all range" physical that worked remarkably well in mid ranks (especially against energy mechs)

Obviously you can substitute another frantic if you want, or reckless plus night eagle, but this one worked very well against middle rank energy players (assuming they didn't have a krap ton of premiums)
Untested Dual Nightfall
 

I have not tested this, but I have faced this in the arena once or twice.  I can't remember who played it, but dual nightfall is genius.  Night eagle helps keep people out of frantic range, and back breaker is only used to knock people into nightfall range.  So this is a testing build.
Once you get premium resistance, you'll "graduate" out of this "middle ranks META".  (This isn't META, by the way.)  But it was an idea for people who want to upgrade stuff.  The avatar style boiler only gets better with premiums, and can beat some of the hard RAID and gold portal levels.  As well it will help you beat 1v1 campaign.
Everything else is just your "average" example.
Again, this isn't META, this is just a helpful idea post.  I have tested all of these except the dual nightfall and they work great in middle ranks.
EDIT: some caveats though, really quick:
First, I only run rounded in the arena, that's just me.  I don't play the whole game of running soft counters and then trying to use matchmaking.  This means that I had a bunch of maxed modules, so when I got MPV and some defense matrices, I literally shot up from rank 7 to rank 4.  Rounded is just how I roll.  Not saying soft counters aren't valid, it's just not me.
Second, I would intentionally run a randomizer in the arena in terms of what mechs I picked in middle ranks.  This is because I would get at least 1 win per day of someone who either picked the wrong mechs or tried to switch mechs to compensate.  I was taught if someone switches mechs, I should switch mechs too.  This means one win per day was someone quitting out because they picked the wrong mechs.
If you want to run soft counters or hard counters, this is not the list for you.  Nothing wrong with counters, I'm just not into them.

Hell no

Fixed, huge thanks to @Eneg for finding the issue.

the plasma fortress was worth it, but you shouldn't try this anymore since you could just end up with 5 epic items, not getting anything good. like person above me said, save for very good offers. still nice drop

Second max'd mythical. Now to focus on my physical mech.
 
 

Your tax money hard at work.

Finally.
By the way, why is green paint cheaper than red?

#2 on my main.  i need a 3rd.

They're definitely not the same.  And that 18.33% is less than 20%, hence I'd rather not tell people directly that "every" premium pack "will" contain a legendary.  I'd rather explain to people that, statistically, not getting a legendary is more likely in a premium pack.  But of course, this is several packs over time, so one needs to remember this before flipping out when you get a premium pack that's all epics 😇

Now you can't un-see it.

Some of you may have wondered if building Chance of Legendary in the base is worth it.  So me and my clan, N0ble Warriors, ran some chance of legendary in our bases to see what we came up with.  So far we've run 54 and are going to keep going until 100.
BLUF: Yes, worth it for legendaries, not worth it for premiums unless you're F2P.  Chance of legendary in the base gives more legendaries than the Feb 21 Deal, but fewer premiums.  This is the base chance of legendary data:

LONG STORY: Based on previous study with premium pack rates in February of this year, you get more legendaries.  However, you also have a reduced chance, per 54 boxes, of premiums.  Estimated error rate: 1% or less.  The premium pack study legendary rate was 17.92%, but the premium rate was 6.25%.  This means the chance of useful items in premium boxes is twice that of the February 21 deal.
It's still unknown if February 21 just happened to have higher rates, so if you base this on the previous premium pack study from March 14 of last year, you will get twice the legendaries than this previous study.  In that study, out of 40 premium packs, I got 14 premiums that are worth having.  Legendary rate was 34 out of 200, i.e. 17% (which resulted in a quote of 0.85 legendaries per premium pack).  So the rate of that was still higher, at 7%.
SUMMARY OF ALL 3 STUDIES: you can see how the premium item rate is higher with premium packs but lower in the base Chance of Legendary.  My advice: if you're F2P, build chance of legendary.  During this study, one of my clan mates got an OEMP and I got an LPV.  It's worth it if you don't have wads of cash to throw at SM.

Some of you may have wondered if building Chance of Legendary in the base is worth it.  So me and my clan, N0ble Warriors, ran some chance of legendary in our bases to see what we came up with.  So far we've run 54 and are going to keep going until 100.
BLUF: Yes, worth it for legendaries, not worth it for premiums unless you're F2P.  Chance of legendary in the base gives more legendaries than the Feb 21 Deal, but fewer premiums.  This is the base chance of legendary data:

LONG STORY: Based on previous study with premium pack rates in February of this year, you get more legendaries.  However, you also have a reduced chance, per 54 boxes, of premiums.  Estimated error rate: 1% or less.  The premium pack study legendary rate was 17.92%, but the premium rate was 6.25%.  This means the chance of useful items in premium boxes is twice that of the February 21 deal.
It's still unknown if February 21 just happened to have higher rates, so if you base this on the previous premium pack study from March 14 of last year, you will get twice the legendaries than this previous study.  In that study, out of 40 premium packs, I got 14 premiums that are worth having.  Legendary rate was 34 out of 200, i.e. 17% (which resulted in a quote of 0.85 legendaries per premium pack).  So the rate of that was still higher, at 7%.
SUMMARY OF ALL 3 STUDIES: you can see how the premium item rate is higher with premium packs but lower in the base Chance of Legendary.  My advice: if you're F2P, build chance of legendary.  During this study, one of my clan mates got an OEMP and I got an LPV.  It's worth it if you don't have wads of cash to throw at SM.

Doesn't matter because there's possibly a plan to make everyone use the base. So there will be a day soon when there will not be an option to go without the base.

Both.  One of each.
Dual frantic brute is the epitome of low-effort.  Having both frantic and spartan helps.

Both.  One of each.
Dual frantic brute is the epitome of low-effort.  Having both frantic and spartan helps.

Both.  One of each.
Dual frantic brute is the epitome of low-effort.  Having both frantic and spartan helps.

Both.  One of each.
Dual frantic brute is the epitome of low-effort.  Having both frantic and spartan helps.