Black and White. Sure you have a lot of stuff to do in them and RBY had next to nothing, but in comparision to the 4th gen games their way to small. Don't get me wrong Red Blue and Yellow did have nearly nothing compared to Black and White but they were the first pokemon games out so you can't really compare them to newer ones without taking into account the limitations of technology at that point in time. It could have been a lot smaller, but they managed to make a pretty good final dungeon in the end with some seriously powerful pokemon to battle, so it was okay. Black and White needed at least 5 more medium to large routes and maybe 3 more town/cities to supass RBY.
It's a bit tough to group Sapphire/Ruby and Emerald, and Diamond/Pearl and Platinum together, considering their post-games were very different. RS/DP had only the battle tower (As the main feature/goal of course) and Emerald/Platinum had the entire Battle Frontier.
Also, it would be good to include FR/LG because it added a whole new aspect to the post-game, (And partly in the story line) the Sevii Islands... You might say the same for HG/SS, but it was basically the same as GSC except for the Battle Frontier...
Sorry to be picky, but it's just some food for thought...
Anyway, I'd have to go with Black and White. Don't get me wrong, it's a pretty nice post-game with plenty to do... But it didn't have any goal or special appeal to it...
And honestly, I'm sure we could get into the post-game towns without fighting the Pokemon League if we just had some Hot Tea...
Like RedCharizard said, you have to account for the lack of technological capabilities back then. There honestly wasn't much room for a lot of post-game content, unless you'd rather be playing the game on a disc with a controller.
Gold Silver and Crystal had the best post-game in my opinion, because it let you visit an entire other region! That was simply brilliant.
Sapphire and Ruby had the Sky Pillar and the Battle Tower, which at least gave you some reason to keep playing after you beat the Champion, and Emerald had the entire Battle Frontier! (Major breakthrough if you ask me)
Diamond and Pearl had several new areas to explore, those being Snowpoint Temple, Turnback Cave, and the 3 Areas. There was the battle tower and the whole little island area was really cool and unique, along with being able to re-battle gym leaders and capture Heatran on Stark Mountian. Platinum improved the Battle Frontier.
Black and White was great too, but not nearly as engaging or exciting as the other post-games... And once again- Hot. Tea.
RBY was the first series of game so "Post-Game" as it is, wasn't really started until 2nd Gen with Gold and Silver.
I would have to say R/S had the worst post-game just because there really wasn't anything to do, everything was explored by that point, while B/W comes in a close second with its Post-Game and Post-Game area just not making any sense, imo.
Why anyone would vote for BW is beyond me. Compare it to the original post-game scenarios in RBY and RSE, it's brilliant, and could only be thought as otherwise in the mind of an eleven year old ingrate. Remember that the third version in the BW series has yet to be released, which will greatly expand on BW's post-game.
I bulldozed in and voted for RSE, mainly because I originally considered FRLG instead of RBY, the former of which majorly expanded on the post game. Even still, RSE remains to have one of the weakest post games, which is a bit of a shame considering it was released directly after the franchises' largest post-game. BW was far more extensive.