The only WDW ride I would not recommend single rider for is Rock N Roller Coaster. I have waited longer than the posted standby wait multiple times in the single rider line so I've learned my lesson there. A good rule of thumb is that if you can physically see the single rider line end on the ramp towards the inside queue area, don't get in line. Just hop into standby or get a
Lightning Lane.
Also Space Mountain doesn't technically have a single rider line but when you are in the room with the standby/
Genie+ merge point, if you tell the CM you are a single rider they will send you to a different line.