Chuck Berry is an excellent guitar player- you just wouldn't know it from what he does today. Like BB, he's a trailblazer, heavily influenced by T-Bone Walker. Listen to his old Chess sides from the 50s- Not just the pop stuff, but the B-Sides, like "In the Wee Wee Hours". He's an angry, bitter man, and has good reason to be (I won't get into that here). That doesn't excuse his onstage behavior. I saw him at the Rockport R&B Festival (which was a benefit). He was the closer. Showed up late, got up on stage, plugged in, and didn't even bother tuning up! We walked out on his ass.
If you want to see just how good he can be (and what a pain in the ass he is), check out the concert film "Hail, Hail Rock and Roll!", which is his 60th birthday party concert. Keith Richards is the musical director, and at one point they come to blows. Great stuff, and the actual concert ain't that bad either (Johnnie Johnson, Richards, Clapton, Cray, Etta James, et al). Can't wait for it to hit DVD.
In the pantheon of blues players, Freddie King is THE MAN. Clapton still wants to be him. The fact that he doesn't appear on this list shows how little the folks at Rolling Stone know about the history of American popular music, let alone The Blues.