"Stubby" Voltaic SMG
Hyperalloy Assembly
Breach Cutter
Light-Weight Cases
Platform Gun
LMG Gun Platform
Engineering Suit
Pickaxe
S.S.G
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.