speed isn't everything, unless your name is Michael Angelo Batio.
If I had to pick one guitarist who was "the best" I'd probably have to go with Steve Vai. He can play incredibly fast, without having his music sound "robotic"
The man seems to be able to channel raw emotion into his guitar, without having to sacrifice speed to do so. Compared to other superfast guitarists, this makes him a rarity.
If you ever listen to Yngwie Malmstein(probably misspelled), you'll notice that his music sounds pretty emotionless..but very fast.
Michael Angelo Batio, is in my mind, the fastest guitarist in the world. He uses a rather unique picking style that not only allows him to play a million notes per second(figuratively speaking) while somehow being able to keep his picking very clean and crisp.
John Petrucci(probably misspelled again) is another really fast guitarist, but he is INCREDIBLY SLOPPY, at least when compared to a guy like Batio.
If I had to pick a single guitarist who could compete with Batio's speed, it'd have to be Symphony X Guitarist, Michael Romeo.