>>23355145XY and ORAS aren't the definitive games. These older games all provide something those ones don't.
Want the definitive Kanto adventure?
FRLG (though I'd argue HGSS is all you need)
Want the definitive gen 2 experience?
HGSS (I'd also argue it's the definitive gen 4 experience.)
Want the definitive Hoenn experience?
Emerald is kind've that. It mixes elements from both Ruby and Sapphire and has a Battle Frontier you could literally sink hundreds of hours into. Whether ORAS is the true definitive Hoenn game is up for debate.
The definitive Sinnoh game is of course, Platinum.
BW2 being the better set of Unova games seems to be widely agreed upon here, making them the definitive gen 5 games. (I'd argue you need to play gen 5 as a whole though, not just these.)
The definitive gen 6/Kalos experience is almost no doubt going to be the third set in gen 6 (X2Y2?) They can only build upon what's already been set, so there's no reason to argue XY are definitive games when you know a better one is going to come out soon.
