See here's where we disagree. I don't think that it's fair to say that since clapton and vai both play with emotion (a heavily subjective statement) that they cancel out, and vai's technique makes him better. They are different guitarrists, Vai definitely has better technique and is therefore better at shred, I'd say clapton is better at blues/blues rock. Vai (and this is just my feeling) has never struck me as the kind of guitarrist who is very aware of melody. He's capable of melody making, he just usually doesn't do it. I can say that I've never heard a memorable solo of his, they are usually either cramming max notes into minimum time, or makign wierd sounds. Keep in mind I own passion and warfare, and the only song that I think is a real song, with melodic development and ideas, is sisters. Otherwise it's basically a bunch of wanking centered around a riff. I'm not saying I love clapton either, though, they are just different. It's not that I hate that style, I like Joe Satriani, Vai I just feel doesn't make cohesive music. (If Ekstasis reads this he'll be all over me.)
I think if we use technique to judge, then we are forgetting that the point of music is more than mechanics. If we're to judge Django Reinhardt against, hmm, lets say jeff beck. It's not fair to say "they both have emotion, beck's got better technique, he's better." Jeff Beck would never agree to that statement, sure he's got better technique (Django only had 2 fully working left hand fingers, that's limiting, though he did get around it pretty well) but his improvisations were incredible. It's Jeff Beck's opinion that Django was the best ever. While like I said, I dont believe that anybody is best ever, I don't think that there can be anybody who is objectively better than django. It's impossible to say that "his improvisations are more melodic than django's" because it's totally subjective. That's why there's no best.