Earth - Duel Gats and Sniper Rifle are fun as hell to play with, but I have yet to master them....still love this class
Doom - My favorite game of all time encompassed into a playable class...nuff said
Slipgate - Explosives are the best of all classes IMO
Strogg - Q2 also has a special place in my heart....mainly because of the Chaingun and Railgun...BFG is fun too :p
Arena - The most well rounded player class with a little bit of everything
I love all the Generations and have always been (and will always be) a big fan of this mod and its predecessor from Quake 2. When I play, I typically choose Random first, and if I start losing pretty bad, I switch to Doom or Strogg