I don't think there's much of a difference with regard to altitude, so it's a tough call. I'd probably put Snowmass on the bottom of that list for this year, possibly followed by Solitude, with Big Sky being on the top.
Snowmass, despite it's northern exposure, seems to go to crap fast in the spring.
You never know when Big Sky is going to turn to slush. When it does huge sections become unskiable. The good news is a large amount of the mountain still holds well due to its sun exposure and/or altitude. You can always find someplace to ski there in the spring.
When considering Big Sky you have to take Moonlight into account, also. Its Headwaters Chutes are all north facing so they rarely slush out. If you are looking for extreme terrain you can't beat them in the spring. Much of the remaining mountain holds well, also, but unfortunately you need to ski down into the flats to get back on the lift and that can be brutal.