England, so far. Very well balanced (the extra spy in G&K is so clutch playing on emperor/immortal since I often find myself falling behind in tech right around the Renascence, and later use it to control one more key city-state so that I can ignore Patronage policy in favor of Commerce). I think overall, I prefer range/movement bonuses on my UUs as opposed to higher unit strength numbers, since it really lets you plan out more advanced military strategies and abuse the terrain. England has both (movement in seas, extra range in longbowmen + upgrades).
Haven't played Mongolia yet (saving it, since historically it's my favorite civ), but I imagine I'll very much enjoy the Keshiks/Khan combo =D.
One Civ I tried and I was surprised that I didn't enjoy was the Aztec. There's just not enough enemy units in the game to truly take advantage of the UA, and so many cities in the way, slow down my progress (lose 2 turns at least to take each city; not to mention the ensuing happiness juggling). Building Jaguars also really hurts your early game infrastructure. Meh =/