I'm guessing either Diamond or X, possibly X. I believe they both have around 450-500 hours. I did a lot of breeding and trading in those games, so I suspect that inflated my hour count a bunch.
I have a bunch of games in the 300-400 hour range, some in the 100-200 hour range, and then a few under 100 hours. I haven't put very much time into the newer games (I don't think I've exceeded 100 hours on a single game since X; the closest is probably VC Gold so far, around 85 hours, though I had about 260 hours on my old cartridge file for Gold as well).