I like TUWP a lot, it's a good score & not something that's ever likely to be performed again (or at least very seldom), but my favorite non-guitar album is his
Piano Reductions (one YouTube previously posted here), which is technically a Steve Vai album, with Mikey performing the Vai compositions on piano (as I said in the OP he's an even better piano player IMO than he is a guitarist, and he's fecking unreal on guitar).
Another favorite, Wooden Smoke, his quiet, autumnal 2001 album, along with the accompanying CD of alternate versions of many WS songs, Wooden Smoke Asleep