There aren't that many Single Rider lines at WDW and they can vary in efficiency:
Test Track - This is one of the best SR lines because of how the cars fill. It is usually very quick, even if a bit backed up.
Rock'n Roller Coaster - This is one of the worst SR lines because of how they fill the pre-show rooms. I would avoid it and just wait in Standby or get a FastPass for it.
Millennium Falcon: Smugglers Run - Reports vary but I didn't like this. With not that many people in front of us we saved only a little bit of time, though this was before FastPass was operating. You will also almost definitely get Engineer as your role if that matters to you.
Expedition Everest - one of the better SR lines as it moves pretty efficiently. The official entrance is a little bit hidden, but look for it between the buildings out front.
That's all of them really. Test Track is a sure bet, and EE is usually just fine (though EE has a cool queue so you might go through it once). Also note on Test Track that you can't design a car in the SR line but you can do a "quick pick" if you want. You can always stop by the kisoks at the exit for the full design experience.