Hardest one I've been able to do is probably Under A Glass Moon. Used to be able to play it pretty close a couple years back, but I've had a lot less time to keep my chops up since then.
These days? I can still play Tornado of Souls and The River Dragon Has Come probably 80-85% correctly... being in university really sucks the time out. I've spent a lot less time learning other songs than I have writing, so the hardest thing I've learned lately has probably been a tricky tapping lick I wrote for one of my songs
Been learning Isolated Incidents by Animals as Leaders and there's a run at the end of the second solo before it goes back to the chorus that's just ridiculous. To to mentioning going straight into a very fast tapping bit immediately.
The only thing I've learned since my post on Page 2 is Guthrie Govan's Wonderful Slippery Thing. It's not so technically hard as much as it is just hard to get "it". I can play the first minute and a half of it probably with my eyes closed, upside down, and naked in the middle of a crowd, (this is all a lie, but illustrates the point) but I can never for the life of me get it to sound like Guthrie does.