I haven't had time to really get to know all the character classes by using them. For instance, the medic is the class that I've used the least (nonsupporting jerk), next would be the scout, then spy. I've earned the most points with the Sniper, but he only has about 45 minutes play time. The most I've played as is the Engineer. Sitting around, waiting in the basement of 2fort, turret maintenance, dancing, peaking my head out the door to see any action.... I sit behind the desk admiring my turret. A fellow engineer comes in to admire... Hmmmm... thats strange, this guy has the exact same name as I do... He walks straight up to my turret and attaches a sapper, then I chase him around with a wrench, then repair my turret, then he puts a sapper on my dispenser, I take it off then chase him around the room some more. I clunk him on the head with a wrench to find out he's a spy.
To me, he is the most annoying character class. I get much satisfaction in killing them with my trusty wrench. Yeah, I know, they're useful when they're on your side. But I just hate it when they have me chasing them around the room as they attach sappers, leaving everybody else confused. I need to try playing as the spy. But I just love to kill them with my wrench. Especially when they lose their disguise in front of my fully upgraded turret, painting the room with blood and body parts.
So what is your most annoying character class? I'm sure everybody has one, depending on which character they play as the most.