My favorite way to play Civ V is Babylon on a large continents map or the Incas on a standard Highlands map. As the Incas I can kick some serious ass on that Highlands map. It's almost like the Incas were designed for that map. I can't think of a more civ to map specific way to play and take advantage of a Civ's UA, except for maybe the Polys on an archipelago map.
So it would be my suggestion to pay for at least the Incas, but also get Babylon. I've bought them all, but don't even play with all the civs I've purchased. I did play the Polys for a while and they were fun. The rest of the DLC civs are sort of meh to me.