Interesting question that is being passed around quite a bit. Even more so since it seems many of them are actually getting some TLC of some sort or another.
I would say for REHABS (i.e. go in an spruce everything up but no majorly changing anything)...
Space Mountain (darken it up)
Haunted Mansion (audio especially)
Snow White (just update some elements)
PPF (same as SWSA)
El Rio del Tiempo (update and redo the final scenes)
Canada and AA ending films (update)
COP (although i would almost rather see it migrate to Epcot or DL)
as far as Updates...
All of Imagination, especially JIIwF - just needs to be fleshed out to be a full attraction on the ride, better use of the vertical space, even if just visually, and a new 3D film)
SSE - coming in the near future almost guaranteed
TTA - new scenes in the tunnels
Speedway - (new cars and perhaps new tie into Mickey's Toon Town Fair)
Kali - Add some new elements.
Star Tours - redo the concept, perhaps adding the multiple destinations and new generation display versions
This is a long list that would bring everything up to date. Top ones would be Space Mountain, the Epcot Films, Star Tours, and Imagination.