Dragon Challenge is quite a walk. Just so you know. Also, I recommend that you hit that when it's a low wait time, as you're going to be possibly waiting more for a locker than in the line (if it's a higher wait time), as there isn't enough of lockers to accommodate the amount of people who need to use the lockers.
As for single rider line rides, FJ, Mummy, Spiderman, MIB are usually not that much of a wait.
I can't tell with Hulk and Dr. Doom, as I don't go on Dr. Doom as it hurt me once (besides I don't like the ride experience anyway) and as for Hulk, I haven't been in the single rider line since it's been renovated.
Gringott's and Transformers are a toss up, sometimes it can be short, as sometimes people abuse the single rider line and it can get long when it's past a 30 minute wait. It can happen to any ride, but I notice it happens the most on these two rides in particular. However, if it's past 30 minutes, do not abandon it right away, and still check the single rider line. However, if it's past 45 minutes, it's definitely likely that the single rider line is packed, or might be closed.
RRR, I do not ride often, but what I can tell you, is that you should not necessarily trust the wait time for what it says for single riders. Last time, I went it was "10 minutes" and it was well beyond that.
Either way, my tip for you is that if you're in a single rider line, if it's more than 15 people, 20 tops (if you want to be really patient), just try again later. I'd give general descriptions of where it would be good to turn away in the queues (when it's too much), but it would be to complicated (or too long) of an explanation, so I think it's best to use people rather than location.
As for general queues in the parks, I cannot think of "short" one's off the top of my head, they're all a pretty long walk, depending on how many people they are expecting to be in the queue. But now that I think of it, the shortest one I can think of off the top of my head however is JP.