I have had good and bad experiences with both single rider and FP. I like to travel solo but did bring my mom along one year, but still ended up doing rides by myself since she's not into those. I love the single rider line for RSR- earlier in the day its great, but as it gets later the line definitely builds. I still pull FP as well just incase the single rider line gets too long I can still get in a night ride! Matterhorn single rider is the only way I've rode the ride, however last visit they did close the single rider line for a bit, and I was told to come back in an hour or so. I imagine the line was full or something, but no clue why. But when I came back I was the only one in the single rider line and waited less than a minute. Splash single rider i've had good luck on, usually only a couple people in line in front of me, but I usually ride the ride earlier in the day closer to 10-11. However that being said, i've never actually used a FP for that ride. I always end up pulling one for late in the day so I never use it. The single rider for splash is accessed by going in through the exit which I think must confuse a lot of people, it sure did me at first. Also every time i've done single rider i've sat in the very last row. Not sure if this is planned or by chance, just a heads up. Screamin single rider has taken awhile before last year In August, but its easier than waiting through the full line. Not sure about FP waits. The ride is great to ride first thing in the morning through the regular entrance, I've seen cast trying to pull guests onto the ride it was so empty. GRR Single rider is typically a very short wait, except last year in August it was HOT, and the line was decently long. SOarin single rider still takes forever, just as long as fast pass. I haven't rode Indy single rider, but FP was never too long of a wait, just a log of walking to get there.
Hope this helps!