Personally I'm dying to see Rick Hawn vs Michael Chandler, that's about as legit as you get outside of Zuffa.
Aldo vs Edgar is a very big fight for the UFC IMO.
But let me pick one that hasn't been made. A big fight that will never happen would be Daniel Cormier vs Cain. But I guess I'll keep going and find a fight that hasn't been made and can actually happen.
Gilbert Melendez vs Bendo could be huge. Pettis vs Bendo 2 would also be a big one.