Most influential in rock? Answer probably depends on your age. But going back in time and looking at the big picture, my top 5 in no special order:
1. Chuck Berry
2. Buddy Holly
3. Eric Clapton
4. Jimi Hendrix
5. Eddie Van Halen
Honorable mentions to Randy Rhoads (still so influential that entire guitar magazine issues are dedicated to him), Tony Iommi, Jimmy Burton, Peter Green, Link Wray, **** Dale, Y. Malmsteen, Paul Gilbert, SRV, that crazy bucket person who was raised by chickens, and a hundred other great guitarists.