This is a classic example of trying to compare "then & now". Its really not possible.
If I could pick one back to have for the Seahawks in his prime it would be OJ. Wouldn't most of us? OJ was the hard running type like Lynch who hit the pile, hit the pile, hit the pile, and one time came out on the other side of the pile and ran away from everyone for 40-50 yards. And remember he broke Jim Brown's single season record in the same number of games per year 14, not the 16 Dickerson had to barely edge that record. And he is the only one to break Brown's record of 7 consecutive games of over 100 yards rushing.
And there are others. If this O line could run decent sweeps I'd want Paul Hornung the best sweep runner ever, followed closely by Gayle Sayers. For straight speed runners no one compares to Sayers.
But we can't have those people. As has been said, Alexander was the league MVP that year so for that year at least you nay sayers have to admit to his value. (but probably won't)
Lynch is great for us now. His running style worrys me greatly as I go back to the 60s and can tell you that most backs that love contact like him have pretty short careers. I would love to have a really good O line for him to run behind so he won't have to take so much contact.
So why do we have to like either one "best". I loved Shaun when he was in his prime, I love Lynch now. Why isn't that enough?