I personally do not like playing as Engineers because I really find their combat drone ability useless and they do not have much in the way of actual combat. Now they could be useful against Synthetic enemies but things are alot different when they are fighting organic enemies. And their defensive capability is also not that good and the tech bursts are cool and all but not nearly as good as the biotic combos. I think Sentinel is a much better class, because it allows you to mix up your tech and biotic abilities and the gameplay and combat is also well balanced. My second least favorite is the Infiltrator. Now this may tick some people off, but lets be hones,t there are not really any stealth missions in the series and you are not really sneaking your way through enemies in anyway, they always know you are there and the encounters are always shootouts. Sure you could use the cloak and take out enemies from a distance or sneak up behind them, I just do not find this class as fun to use as the others. Now just because I do not like the Engineer and the Infiltrator classes it DOES NOT MEAN that they are useless and flat out bad that no one should be playing as, it all depends on one's personal preference and play style and I prefer the other classes.