Agree totally on Hangar 18, was going to mention it myself but I thought I would hear a few replies first. In total it is something like 11 solos but as a whole it is a masterpiece.
One of my favourite solos has always been Lucca Turilli's solo on Unholy Warcry, I am not a big Rhapsody or Power Metal fan but I that solo is genius.
It may not be the greatest guitar solo of all time but the shred guitar on Slipknot's welcome is pretty tricky, I learn't to play that and it is not easy (for me atleast) with 32nd notes all over the fret board.