Cho-Ko-Nu, because they're amazing:
2-3 first strikes
50% bonus on melee
does collateral dmg
this is all before your vassalage/rax/etc upgrades
Tell me that doesn't kick a little butt.
Marine, because I'm an evil backstabbing bastard:
We all know that your greatest ally midgame will probably turn out to be your greatest enemy lategame (Diplo or Space-Race), and that's why I like to keep about a dozen transports brimming with marines. I can level about 5-6 cities in one round, and completely destroy any civ's chance at a win, should I choose. Of course, only a couple times did I go on to win myself, after that. But nevertheless, razing 5-6 cities in a single turn and losing an equal amount of marines (usually the only ones that die are the ones who have to actually raze the city) makes me smile.