Done a stats reset but overall I played most with a pyro or engineer, and least with a sniper.
Pyro: Fun fun with backburner on goldrush defending

. Though less fun when the server is too full or too empty, best at around 10 per team so 20 players imo.
Soldier: A bit meh but enjoyable once in a while.
Heavy: A bit meh sometimes but enjoyable in a while: Bloody spies
Sniper: Not fun, bloody spies&snipers
Spy: Fun backstabbing or shooting heads with ambassador @ annoying snipers, engies&sentries, Heavy's&medics

Medic: Fun using the ubersaw on people

and generally plays well due to the shortage of medics. Awesome to play with a mate as 2 medics, medic ownage

.
Scout: Fun until you come across a large group or a sentry, best on the more empty servers imo
Demo: Fun but also meh sometimes, sometimes I play very well sometimes I suck playing as one.
Engie: Fun but spies, demomen and ubers are really annoying, and sometimes don't have the patience to wait.