Pyro. W + M1 FTW!
Most of that is because I really suck as the other classes. Scout would be my close second, but Pyro really suits me best.
I have a strange backburner, a strange flaregun, and I'm good with whatever melee weapon, I never use it anyway.
For the entire round, I play sneaky, but when caught I can hold my own. I find the Spy is far too weak in a straight up fight, and the knife to be far too finnicky about when exactly I'm aiming at their back. The Pyro loadout I have allows me to cause massive damage from behind, spycheck EVERYBODY, cause good damage from close range, light people on fire at long ranges or crit on people who escape short range, and if worst comes to worst and I die without killing the person, the damage over time sometime nets me a kill.
I get that it's a team game, and that I should be more focused on points as opposed to kills, but I find that kills are tangible, reliable points, whereas playing medic nets me points only from people who kill things for me, or as a scout I generally only get points from those control points (which are inevitably rocket-magnets or guarded by a level 3 sentry, so why bother?). I prefer to help my team by ensuring that the enemy team spends more of their time respawning than capturing objectives or killing my teammates. Every dead enemy is one less guy who could be winning the game for his team.