"Stubby" Voltaic SMG
Breach Cutter
Platform Gun
LMG Gun Platform
Engineering Suit
Pickaxe
Loadout Guide
People have missed some of the strongest Engineer builds. The SMG is by far his strongest and most ammo efficient primary when built correctly. This build can deal with every enemy, use Breach Cutter as listed or Shard Diffractor for armor break (you don't need to stun what is already dead) :) If using Shard Diffractor use Plasma Bursters for burst dps over S.S.G's.
Overall this is probably the 2nd strongest Engineer build. I have another that I think beats it but would need to do more a/b testing.
For passive perks you want resupplier, born ready, and any other of your choice (recommend thorns or deep pockets).
For active perks dash is always mandatory and heightened senses beats iron will as you shouldn't be dying with this build on non-modded difficulties. Iron will is one and done whereas heightened senses alerts you to (often RNG BS) danger through the whole mission even after it's two kill activations and keeps dps up in critical moments if and when you do have to use it.