I'm just starting to get serious with Civ V (have logged 100+ hours in the past couple of months). My favourite is smaller map (the AI turns take too long on larger maps), and large islands. The islands make war more fun, as you can sail to most enemy cities with your fleet and don't have to crawl through the landmass. Also, once Frigates are available, the land units become largely irrelevant, but the AI doesn't really comprehend that, which gives you a big advantage.

I play as Phoenicians, as they get nice bonuses for a water world.