Of his era, probably. No, definitely. But hard to compare eras, really. Every stage of GP racing has had it's advances in technology and style, with the best riders coming to the fore in being able to take advantage.
The best I ever saw was Mike Hailwood, and he had some pretty stiff competition in the likes of Redman, Ivy, Read, Ago and the like. I sound like an old fart, but perhaps the depth of talent was greater in the 60's and early 70's (?). Although if you think about it (what, me think?!) not a hell of a lot separates the field in a MotoGP race these days, everyone is bloody fast. Rossi's domination ('cos that's what it is) is the more impressive when you look at the diversity of results of the riders behind him, there's no real '2nd best', although Gibernau last year could be termed that, but certainly not this year. Melandri, maybe?
Next year will be interesting, but I rather doubt Rossi will be dethroned until he gives up 2 wheels. And that will be a sad day for GP.
Cheers
Barry