Damn, I was gonna do a poll on this. Oh, well...
Here are my fave units from various stages of the game:
Phlanx: Good defence, esp. w/ city walls
Knights: Good combination of mobility and fighting ability
Musketeers: Good on the offensive or defensive, esp. in numbers!
Marines: Ah, yes. I rarely use amphib. assault, but they chew enemies up & spit 'em out!
Howitzer: Negates city walls AND has an attack power of 12? Need I say more?
Battleship: Nighty night, enemy navy. Hope Davy Jones' Locker is nice and comfy...
Nuclear Missile: I don't really need to explain my love

of nukes, now, do I?
Stealth Fighter: I like these for multiple kill ability. I kill as many enemies as I can with these & I don't care that they run out of fuel! I LET them! Kamikaze pilots! Nyah!