Well, if you think of "elite" as in very powerful, then M2TW has a lot of them. Order knights should absolutely not be any better, they are IMO too good right now, as there is no need to get any other knight at all once you have them...

As for powerful units; you have janissaries, most late knights, the awesome dismounted order knights in the levant, Camel Gunners, Conquistadors, Cossack Musketeers, Dvors, the list goes on...

If you mean elite as in very rare, then I can think of only one unit, really, the dismounted order knights.

EDIT: Oh, and the elephants, of course...