3 songs immediately come to mind (and 2 of them feature Buckethead

) I can't possibly say which is my favorite. There's too many to think of that aren't mentioned here, but these 3 pop into my head immediately.
1. Buckethead, "Nottingham Lace" -- This is a 6 minute+ rock instrumental, and within the category of rock instrumentals, this has to be in the top 3 ever made. This song kicks so much ass, it's hard to put into words. Buckethead takes you on an adventure with his guitar-playing, and this it's all done with good straight-forward soloing (none of his weird computer-style super tapping or any of that weird stuff he does sometimes). It's got good structure and beautiful melodies throughout. This song is the definition of epic.
Listen to it NOW
2. Claypool's Bucket of Bernie Brains, "Buckethead" -- Yeah, the song is called "Buckethead", go figure. I love this song for a few reasons aside from the solo. Claypool always cracks me up and this song is no exception. First half is classic Claypool: awesome, heavy, funky bass with a great band backing him. Bernie Worrell delivers one hell of a keyboard solo right before Buckethead does his thing. The whole song seems to lead up to the guitar solo and the guitar just basically explodes into the second half of the song. Mega-epic, just like Nottingham Lace. Can't find it online though, otherwise I'd post a link. Check it out if you can.
3. Steve Vai, "Tender Surrender" -- Well, it's Steve Vai. Personally, I think this is quite possibly his best solo and one of his best songs overall. Starts out soft and melodic, really a beautiful song. It grows and grows and slowly turns into a screaming Steve Vai face-melter. There so much emotion in his playing, it sort of tells a story even though it's instrumental. Vai has that incredible ability to make his guitar sound almost alive, and that really shines in this song.
Listen to it NOW