My highest hour counts are on X and Diamond. Both have somewhere between 400 and 500 hours (I think closer to 500), but I don't remember the exact totals. I've never restarted either one of them, though, and most of that time was spent on breeding (especially on X) or on trading-related activities.
The only physical cartridge that I've restarted on is Red, which I've had three main files on (four, if I count one I restarted very early on, plus another on on my eShop version and some other non-cartridge ones). I don't know how many hours I have on RBY, but I know none of the individual files have gotten up to the 255:59 max.
The game that felt like the largest time commitment, even though it wasn't, was Gold (which died at 257 hours). I think time felt like it moved more slowly back then, and I have a ton of memories of playing that game, so that's probably why it feels like the most meaningful. I only have 2-3 hours so far on my eShop copy, though, and I never restarted the game on my original file (except for a quick couple of hours after the battery died and my save file got deleted).