The TGP arp solo is the first one that comes to mind, but the difficulty of that solo is just the speed and precision. But the fact that JP struggles with it live is proof that it's a tough solo.
I recall that Jordan said in an interview that JP doesn't like to play the Blind Faith unison because it's hard to play on guitar, so that's a contender (although it doesn't sound as hard as it probably is). I guess the trouble with that is the scale or the shapes?
There are a lot of solos that are difficult due to speed alone, such as the ITNOG instrumental section, the TDEN guitar solo, the first guitar solo in Beyond This Life (which as I recall is entirely purposely out of key, which may make it difficult pattern-wise), and Constant Motion.
Then there are other solos where the difficulty is the technique used. I'm thinking stuff like the UAGM solo, with that weird sound he does by swiping at the strings (no idea what it's called or how to describe it). I think there's also some funky whammy bar usage in that one.