I haven't been watching Pokémon at all ever since Black and White series had begun; somehow, it tended to chase me away. Their art-style didn't seem to catch my taste very well, most music were slow and steady (which didn't seem to complement their usual way of approaching things), and in general, the plot seemed to have been rather bland. However, I've gotten some chance to watch some episodes of X and Y during last couple of months, and thankfully, it has definitely brought my interest back!
As for which Pokémon I'd like to see Ash bring back, I'd say Swellow from Advance Generation series. Apparently, they currently have seven unannounced mega-evolutions in ORAS, and I feel it's very likely one of them might be Mega-Swellow, especially considering that it's native to Hoenn and is seemingly more popular than some other bird Pokémon they have. If they do introduce it, I believe this would give them an opportunity to show it all off in-anime.
Aside from that, Sceptile would be pretty cool, too, as mentioned above. If they'd brought Gible back as Iceman had stated, it wouldn't be so bad, either, as we'd have some chance of seeing the Pokémon go through its evolution and whatnot.