if i’m in a moderately serious server, then I prefer the spy because I can be the goofball that everyone hates 
… I can go behind enemy lines and have fun that way (and he’s got a knife ), or I can cloak up as my own team just to make the other team’s rushers pause for a few when they see another heavy / engineer / sniper / demo in their way 
now, if it’s just a server full of people doing stupid crap, I like playing the pyro for his mad air guitar skills and amusing (and now deadly) kamehameha-like taunt.
and for farming out achievements, I like to be the spy and just stand around and burn my way through a bottomless case of cigarettes… Dance Dance Immolation and Got a Light? are the fruits of torching me while burning down cigarettes 