I've yet to play extensively at all, but from a(n impossible-to-beat*) test with singleplayer, Engie looks to be my favored class. While I quite like that during setup, all buildings automatically start at level 3, I suspect I'll end up rolling the Gunslinger and play Battle Engie. My reasons boil down to a pair:
- The Engineer has several upgrades that enhance his metal pool
- Piercing Widowmaker
*It should be noted that, while literally impossible to beat wave one with one player, I still had fun getting my ass handed to me by all those robots. I s'pect I'll be playing MvM for quite a long time.