The NBA game is always changing, evolving, and systems/teams must adapt to it.
Howard's speed/quickness/mobility with his strength/size/athleticism makes him the best Center in the league.
He is actually 6-10, but is very strong/athletic.
He is mobile like a forward, which enables him to be the best help defender and can recover quickley.
Although Bynum is bigger, much more polished offensively, he simply is not even close to the defender as Howard is, even though he puts up similar numbers with rebounds/blocks they are completely different.
Noah would be 2nd player with legit size.
Not only can Howard/Noah both defend their own positions but they cover so much ground with their help DEF.
Gasol brothers, Cousins, Bosh are the 2nd tier, KG *even though old* deserves to be in there as well.
Ibaka, I am not sure if you can really consider him a C, I consider him a PF more, while still improving and much upside.
Shaq didn't even draft Bynum and Lopez in his fantasy basketball league, although it is a 6 team, 8 roster, and 9 stat league.
Proof that he is just bitter when it comes to Howard, leaving out his "best 2 centers" in the league, I don't even think Lopez was drafted.