If it has to be just one character, I would say Cordie, primarily because, as a Higher Being, it wouldn't technically be bringing anyone back from the dead, which Joss has done to...excuse me... death. As much as I'm sure that Faith is going to try, I think that Cordie is the ONLY one who would stand any real chance of getting Angel up off of his brooding a** and back in the game on a permanent basis. Buffy loves Angel too much (see, Bangels, even a Spuffy shipper can say it) to ever be harsh enough on him to ever get any results, and Faith just plain feels too much loyalty and downright devotion to him to ever really get the job done. Cordie is Angel's only real chance to get back on the road to redemption.
That's not saying that I wouldn't love to see Tara, Wes, or Fred come back, but I have to say that, of all of those who have passed on, Cordelia Chase would be the most useful and valuable resurrection candidate.
Besides which, I really want to see her reaction to Angel's naming a dragon after her, and a cat fight between a werewolf and a Higher Being with Ol' Broodie as first prize would be hilarious.
Especially with Buffy and Spike cheering them on from the sidelines.