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.