When assessing seasons as a whole, it is like careers: the end results are what matters, not the eyeball (subjective) determination of level of play. All three seasons are very similar, but in every metric I've applied, 2008 is always third. But again, very close.
So I might agree that in 2008 we saw Rafa at his very best, but that doesn't make it his best season. That's determined by the sum total of the year.
Just as many believe Novak's best form was in 2011, but 2015 was an overall better year.