I agree with PerformerInWaiting, TOT is amazing and I LOVE going on it!
I'm not sure which ride is the absolute most popular, but out of the
four, I'd probably say that California Screamin' is the least popular and maybe Midway Mania is the most popular. That ride is pretty new so it's bound to almost always be packed full.
"Well I don't know about you folks, but it's way past my bedtime."
To me it's usually a toss up between ToT and Soarin'. Midway Mania has the longest lines, though, but that's probably because it's still reltatively new. In a couple years it might be the same as the other two I mentioned.
Id say Midway Mania is currently the most popular out of those 4. Total I'd say Soarin is the most popular. It is the ride everyone talks about.
Tower of Terror is talked about a lot too though, but not too many elderly folk talk about Tower of Terror. Soarin' appeals to all ages.