The list of great designated hitters is a short one, mostly because the list of long-time designated hitters is a short one. You might say Manny Ramirez should be a designated hitter – should have been a designated hitter for some years now – and I might say you're right. But Ramirez hasn't often been a DH, and as long as David Ortiz is Ramirez's teammate, he won't often play that role.
Why hasn't Ramirez been assigned that role already? Or rather, why aren't players like Ramirez more often turned into DHs? Two reasons: One, most players prefer to play a real position, no matter how inept they might be in the field (we humans have a real talent for self-delusion). And two, once you turn a player into a DH, you've knocked down his value a few pegs. If you keep him, you don't have the DH slot available for some other good-hit, no-field player that comes along. And if you want to trade him, you've got to either a) find a trading partner in the American League, or b) somehow convince your National League friend that Joe Slugger's really not so bad at first base (no really, I swear).
So when we're looking for the greatest designated hitters, we really don't have a large group of great players from which to choose. Sure, Orlando Cepeda and Harmon Killebrew and a few other Hall of Famers finished their careers as designated hitters, but I don't think anybody wants to call Carl Yastrzemski a "designated hitter" to his face.