I simply cannot say enough about John Petrucci. He is one of my favorite (if not THE favorite) guitarists. His compositions are complex, his knowledge is immense, his technique is amazingly clean and he is a chameleon of style. You name it, he can play it -- sometimes within the same song.
Dream Theater is packed with amazing musicians who are all at the top of their field, but JP really signifies to me someone who is a consumate professional. I simply cannot plug his stuff enough. If you have never heard Dream Theater, you should first slap yourself. Then you should take the most expedient course possible to acquire some of their stuff. Even if it's not your bag, you cannot deny the amazing levels of virtuosity in their music.
Also available is the G3 Live in Tokyo performance that Petrucci did with Satriani and Vai earlier this year. I have not heard it yet, but I shall soon. Visit his website for more info.