Super Mario Sunshine: Although I do like the actual gameplay too much, I thought the game looked fantastic. I know this is about GC games, but if they released an HD disc with 64, Sunshine, and the Galaxy series that would be AMAZING.
Melee: It would be a great candidate for an HD upgrade. It was one of the first GC games released, (which you'll find to be a running trend in my list) but it's also one of the most polished gaming experiences in both controls and visuals on the system. With the addition of wifi matches, it would be a nice hold-over while we wait for the next release.
Pikmin 1 and 2: Like Melee, the first Pikmin was an early game for the system, but I still think it looks absolutely beautiful. Seeing how amazing Pikmin 3 is looking makes me want these two to get the upgrade even more. <3