nightmare

[heat blast]

should i myth nightmare or wait until i get something good like molten platinum vest or windigo? 

[OKI DOKI]

In reality it is about preferences according parts at disposition to use with your torso. If nothing else it is available you need to keep moving forward. Now, nightmare it is a torso C-M and can be use from start to be divined. Some top  players use the torso at top 10 clans as example below. Some indicate it can be the best torso in the game depending the configuration. Now, people use more now vest because less complicated to set up and resist but night it is high in resist as well. All depend what will you do with it. In my opinion you can max it but I do not recommend if you do not know what to do properly but you still need to keep progressing and later when get the item you need go ahead and switch like the majority had to do in the past including me. We do not know when we will get the item we want. 

[Nefertary Meriten-Mut]

It depends on the modules you have. If you have the new combo modules (heat /energy) you will need only 3 module slots and can increase the HP. But if you use traditional modules then you need more module slots and your HP is greatly reduced. Windigo is a little better, but not much better. And a problem that both Nightmare and Windigo have is that the phys resistance is better, but the heat resistance is relatively low, so be careful with that, because there is a lot of heat build on the Arena.