@Felthry I say Zelda 2 because while Super Mario USA/Super Mario 2 was indeed a bit of an oddball game, its style was still quite distinctly Mario with characters and characteristics that carried over into many other games. Zelda 2 on the other hand... There really isn't much that stayed with the series, and while it's definitely not CD-i levels of oddball, it still stands apart from the mainline series of games.
@Felthry Oh! My mistake! Unfortunately I would have to go with SMB2 then as a personal preference, but I did still have quite a bit of fun with Z2
@Kilroy honestly i was going more for "which of these games (which are both odd ones out in their series) is better" than "which is more unlike the rest of its series"!
-F