Madagascar (my pick): This brings to the table the one thing that Universal doesn't have in Orlando - a family water dark ride. Theme parks can't get enough no-height-limit, narrative attractions that appeal to all ages. Madagascar - if it ever opens - could do that. Yes, this ride loses points for its delayed opening, but I suspect that Universal Orlando might have the experience to manage this project to completion better than USS has.
Space Fantasy: The Ride: Universal doesn't have an indoor coaster, either - a strong asset in Orlando's hot, humid weather. (Update: OK, Revenge of the the Mummy. See below.) Space Fantasy's gotten rave reviews and the fact that it actually works might push it to the top pick.
Transformers: Great franchise. Sounds like a great tech upgrade to Spider-Man. And maybe it need to replace Spidey, too. (See my post in August about Universal and Disney and Marvel.) But it's not different enough from Spider-Man to beat out my top two.
King Kong 360/3-D: Big drop off to this point. Yes, it's a great experience. But while it works well within the context of Hollywood's Studio Tour, the Kong show is too short to stand on its own in Orlando. It'd need a new treatment - to the point where we really would have to consider it a new attraction, not a duplicate of one from another park.
Battlestar Galactica: Universal Orlando doesn't need another roller coaster that doesn't work.
My vote goes to King Kong since it doesn't duplicate anything already in Orlando, such as Transformers does for Spider-Man and Madagascar does (to a lesser degree) Popeye's Bilge-Rat Barges. A pair of dueling coasters of different types in somewhat novel, but Dueling Dragons, or whatever its called in its current HP incarnation is already in attendance. This only leaves the fairly generic Space Fantasy, and that will surely come up short compared to the indoor spinning coaster sure to come to Disney's Hollywood Studios in the next few years. For that reason, King King seems to be the sure bet for novelty.