Out of all the games I've played, Black has been the only one that's ever really got tedious - post-game sucks so bad. But it's not a bad game. The worst game (Although not the least enjoyable) is definitely Diamond. I know it was the first DS main series game, but I'm sure if they put a little more time in they could've had it at least up to Platinum standard. And the slow surfing in particular just seems like a stupid issue, they fixed it pretty easily in Pt, why not just never have the issue?
Doesn't stop me playing it, but it's definitely the worst game.
Haha, forgot Pokémon Battle Revolution. PBR has no plotline, although it never really pretended to be more than just an excuse to see your pokémon in 3D on the TV. And whilst it was good for that, I still had no desire to keep the game - eventually it just got tedious, although I did get an Electivire out of it :).