Well, actually the ex cards weren't the rarest in all of the ex series boosters. That would belong to the Shining/Gold Star cards, which were "secret" rares in those sets, and very rare to pull, though they were only in the later ex sets.
In the DPPt sets, Lv. X was not the rarest for every set. There were "secret" rare reprints of older cards in the four Platinum sets.
In the HS series, there were no Lv. X cards, just Pokemon Prime, which is what corresponded to the Lv. X cards, though these are only indicated by the special foil pattern all Prime cards have, and the fact that their better than their normal set counterparts. HS also had the LEGEND cards (the cards where you need both halves to play the cards) and the Alph Lithographs.
In B&W, so far there is no equivalent to Lv. X/Prime/ex, but there was a "secret" rare Pikachu in the set, and there are full art cards for Reshiram and Zekrom. There also is Pokemon EX (technically different from ex in Japanese because of the fact that one is capitalized and the other isn't) on the way within the next year.