Good point Col, but it's a bit like asking, how long is a peice of string?
Personally my money would be on Rossi, based on longevity and results.
Most of it depends on who's riding what and when.
There has always been some rider who has popped up to win a title, Luchinelli, Uncini, Schwartz, Roberts Jnr and dare I say, Gardner, but when that name pop's up several times over an extended period, that's class.
Mick, coulda, woulda, shoulda won 7 in a row if it wasn't for his leg injury.
Talking my favourite, Speedway, Ivan is regarded as the best but pretty much every year there was someone going better than him but he was always on the pace and was the first real "Proffesional" rider.
Casey is definitely quick, but to finish first, first you've gotta finish.