
Probably the most important one.
Multi-resistant module, 2 cooldown packs, 3 of these bad boys

(which will eventually happen) and that's 728 hp (fully fused) and cooldown/regen that will keep you sorted for all fights. Only a weight of 576 too, so you've still got a lot of room to play around with other items.
oh hey why not add cannon feet with +72 hp and you're looking at a mech with 800 hp that still has the ability to cooldown/regen. At a weight of 645 you also have a lot of room to add weapons still!
The thought of this makes me shudder

Edit: Just read the limitations of mythical items. Even with the torso alone it has the possibility of having 746hp with 10 resistance on all and a comfortable cooldown/regen rate.