Confusing Post Alert!!
That's a hard question. HG/SS definitely has more post game content than any Pokemon game as far as I'm aware. It also has the best post game content, because, well it's Kanto. For any true blue (Or red?) Pokemon fan that's a hell of a gift. In HG/SS versions it's even better than the original because you also get access to a hell of a lot more Pokemon than just the original 251. Not all but a good number of Hoenn and Sinnoh Pokemon become available to you. Collectively between HG/SS I've put in about 210 hours and haven't caught them all (Never will now since I've transferred all of them to Black Version 1).
Also, I don't want to give it away, but there are some very 1-time desirable Pokemon you get in HG/SS you won't be able to get in Black & White.
That's not to say Black & White Versions 1 & 2 are to be sniffed at though. Between them games I've put in over 230 hours but consider the caveat that I've actually trained over 500 Pokemon on them games. But also haven't really touched any post game content in Black 2 yet which it seems as if there's a lot of. Done a couple of bits but I've also got a few more things to do which I can think of. Plus secrets to find I assume.
I think the thing to consider with the new games is they are "better" games than the old ones. The new Pokemon are generally great, the game is a lot faster than the old games and Versions 1 are very concentrated on being a new and core Pokemon experience. Versions 1 post game content is also sizeable, I think. It locks off a 3rd of the map to you until you've beaten the game + there's lots of things to revisit.
Versions 2 feels just like a remixed game to me. They've fleshed it out over the Versions 1 by having lots of content and side things to do but is it all good? Mostly, but some of it feels like filler and ineffectual on what you actually do in the games. It has more post game content, though, for sure and I suppose it won't feel remixed to you if you didn't play versions 1. Out of the new games, for length, Versions 2 is the way to go.
Ultimately, yeah, for longevity HG/SS far outweigh the new games I think, and have some really cool added extras over the original versions of them games. But if you played the original GBC games back in the day it's debatable how "new" they are exactly, right?