Although I would rarely if ever use them, I do agree with the need for more single rider lines at WDW. It's interesting how many they have at DLR (8 total rides) with only 3 at WDW (TT, Everest, RNR if anyone does not know).
The following rides would all be good ideas to add single riders lines at WDW (barring limiting to actually building a single rider queue):
1. Soarin'
2. Tower of Terror
3. Space Mountain
4. Dinosaur
5. Kali River Rapids
You could also put Pimeval Whirl in the list, I suppose. All of them have either odd enough seating or large rows that would often leave single rider spots open and leave opportunities for greater efficiency with single rider.
Beyond that, I think you could make arguments for TSMM, Buzz, and Splash Mountain, but I think they would be less beneficial and be potentially awkward for guests being close to each other.
I see no reason why a ride like HM would use single rider (even ignoring the issue with the stretch room messing up any line). I think it is bizarre to even suggest that.