Haha, Engineer and Medic are two of my favorite classes there (just underneath Pyro, as you can tell from my avatar XD). It's fun whenever I'm medic and healing someone, Spy comes up behind me for a backstab but fails, and he continues trying to do low damage stabs to kill me, meanwhile I just casually crack his head open with a couple Bonesaw whacks before returning to my patient. Never underestimate The Medics =)