i'd go with one of the following, hard to pick.
Soldier
+high damage output
+can fly
+can heal himself (specifically concheror ) and cast several buffs on nearby teammates via the several buffing backpacks
+decent melee arsenal, quite handy depending on the sitch.
+funny mannerisms in general
Pyro
+is a fucking crit machine what with weapons such as the backburner, those shiny flaregun thingymaboobers, and the axtinguisher, not to mention pyrosharking.
+immune to afterburn
+2nd fastest class, i believe.
+almost as diverse as soldier in the field of playstyles
+fluffy cute pyros. 2cute4me @Popcorn
(time for some controversy)
Sniper
+excellent with, well, the obvious sniping.
+5 out of the 6 rifles he uses can deliver instant death fully charged headshots to any class without invuln. (*sarcasm* which one can't? hmmm...)
+sydney sleeper is not, I repeat, NOT a piece of poo. even though it cannot headshot, what it can do is instrumental: no, not quickscoping (#sorrybutno), not even the jarate effect, but the charge rate. with this hardware, you can inflict jarate while doing a substantial amount of health, then mini-crit with your next shot, when you're quick enough. oof. also, thought i ought to mention: sydney sleeper sniper working together with an airstrike soldier? yikes. #thekillfeedtho #theassistfeedtho #thedeathfeedtho
+Jarate. need i say more? marked for death, spies are revealed, and if the sniper has either the bushwacka or the tribalman's shiv, you can kiss your not-a-heavy rear goodbye.
+nice melee arsenal, except maybe the shiv.....