I don't know that anyone here (outside of CMs) could really say, as it's random. Most ride closures are caused by guests, not mechanical issues. Leaving weather out of the equation, I don't know I can single one ride out where I say "Man, that thing goes down a lot". PP mentioned Splash, but in our experience it's very rare that it's down. Not saying they don't experience it, or they're wrong, because they may have seen it a lot. But my point is that it's random. Same for Pirates. We actually got stuck on it once and had to be evac'd after sitting for 40 minutes...but that was one time out of dozens and dozens of times we've been on it.
If I absolutely HAD to pick one ride I notice most often it would be PeopleMover. But even that is sporadic and random. One trip, we saw it down over and over and over, but then others we never saw it down.