As it has been mentioned earlier a quite often, this question doens't make any sense without including Tom Boonen.
They lie in between approximatly 1.5 years. That's pretty much as close as it can get inbetween real top contenders.
So to answer the question who is the greates of their generation you have to look at all the years of each respective riders to do that. If you want to compare top years or overall sucess, it doesn't matter because they are basically of even age.
It has been claimed that Cancellara and Gilbert were better natural talents. I just can't see how you'd come to that judgement. Both of them took quite long to really perform. Booen on the contraty has Monument podiums over the spread of 9 years! And i'm quite sure it'll be 11 after this year. He was third in Paris-Roubaix at the age of 21!!! in 2005. So you have very good arguments for Boonen to be the "most natural talented" rider as well, just looking at the results.
I'm just amazed that people would count out Boonen just because he had two bad years, in which he also wasn't very lucky. There were years in which Cancellara couldn't compete in long TTs, and wasn't taken to the tour because he wasn't a lot of use outside of prologues...
Also it's true that Boonen is a good/very good sprinter in mass sprints (wel'll have to see if he's very good again this year, don't think so though, but who knows), but that shouldn't lead to the conclusion that he's just relying on it! He also soloed to wins and he does attack to make racing hard. It would be stupid of him though if he'd attack when he's the best sprinter in the group. Better to controll the race than to rist blowing it attacking. Maybe this might conjour up the idea of him being only an opportunist.
It's even more impressive that he actually has such good speed, power and stanima. But as I said, I'm just amazed how you couldn't appreciate his talent and forgert him.