I'd agree that Rossi is the greatest ever. But I don't agree that he will neccesarily win this years championship. Lorenzo has the capability to take one or two races from him when everything falls into place. Stoner is indeed a huge threat. No-one else can get what he can out of the Ducati, and as previously mentioned, if Stoner had not been ill for a good part of last year, the end result could have been a lot different.
Spies is just another cat among the pigeons. He obviously has great skill, and I don't think it will take too long for him to come to grips with the premier class bikes. its true that few riders have made a successful switch from Superbike to the Premier class, but in most of the memorable cases, it has been when two strokes have been the premier class. And there is a huge difference between a superbike and a 500cc 2 stroke.
So I would say that to move from one class to another is easier than it once was, and this can only be to Ben Spies advantage.
Valentino certainly has his work cut out for him this season with the younger guys coming through, but all things being equal, I would still pick him for the title. I think the biggest threat will be Stoner, that Ducati is his, and if they get everything sorted for him, he is going to to be very very hard to beat.
Spies? not in 2010, but come 2011, things may be a little different (IF he has the bike).