I think it's kind of a toss-up. SD is obviously the busiest. To me, TSMM and Alien have the same crowd level. But, I find it easier to get FP+ for TSMM throughout the day as opposed to Alien, and Alien is the newer of those two rides. So I'd probably RD Alien and then hit up TSMM. Either way though, the ride you don't do at RD will likely have started to build up a wait.
I think TSMM is the better ride and usually has a longer line, so I'd RD that one.
You could also try to ride last thing at night. Last September we were waiting for it to be almost closing so we could jump in the SDD line. AS2 was a walk-on which we rode 4 times in a row.
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