It depends:
Babylon and Korea are strong scientific civs, athough Korea does not bring a strong presence as an opposing civ (and Babylon doesn't have any type of scenario). Babylon is however a strong presence as an opposing civ and a terribly overpowered civilization to play as.
Spain and Inca are a must, they bring much needed presence within the game, and both Civs have great original dynamics (although very situational). The purchase of Inca alone makes up for the purchase.
Polynesia and Denmark are relative, neither brings a strong a presence, however they are both powerful exploration and warring Civs (Polynesia specially). To add to this, the Polynesia scenario is probably the best of all.
The Explorers pack is a good purchase, you probably won't play as any of the civs all the time, but you will probably play these maps most games, specially since all have better scripting than the original game map scripts.
Craddle of Civ is overpriced and downright terrible, avoid like the plague. Same goes with Wonders of the World, you won't even notice it's there.