well 'best' is hard to define, but as far as sounding the best, pantera's riffs and solos are some of the best. The technical aspect is also another way of determining the 'best', but I bet dimes solos are up there too. I wouldn't nominate the riff from 'walk' for the most technical, but some other songs step it up quite a bit from that.
Cacophony! Hell yeah man. Concerto and Speed Metal Symphony are two songs I am working on right now. I honestly doubt I'll ever be able to play both parts for Speed Metal Symphony though, it would just take too much time to learn all of that. As for Megadeth, Mustaine has written some pretty tricky riffs, Poison was the Cure, Tornado of Souls, Killing is my Business and Business is good, shit...the list goes on and on. Not to mention he had Marty Friedman for a while...
Yngwvie Malmstein is THE greatest guitar player by speed. Absolute no doubt about that. The greatest heavy metal guitarist by far, Alexi Liaho from Children of Bodom. No one can even compare to his style. For all you "noobs" listen to "Chokehold: Cocked and Loaded"
Dimebag forsure, but Kirk is not over rated...he's such a good guitarist, and contrary to popular belief he's not just about speed. But he is fucking fast haha
The best guitarist by speed is a guy named shawn lane. He is scary fast! Although he's not quite metal, he's brilliant, and definitely faster than anyone I've heard (including guys like yngwie).