Uum, I think some people here have it a bit wrong. After filling the national pokedex in black/white you can catch pokemon from older generations in the wild and in dream world.
With both black and white you should in theory be able to obtain every pokemon species in existence bar maybe a couple of version exclusive legendary or event pokemon. (And maybe some dongle pokemon, I can't remember if there are any pokemon you need to dongle in B/W or not...) At any rate, with just Black and White it should be possible to catch, I'd say, at least 90% of existing pokemon in all gens. The only ones I can think of off the top of my head that might be unavailable for capture are event pokemon like Jirachi and Deoxys. But hey, you technically can't catch them naturally in any gen. If you miss the event, just trade those ones, most people do.
Anyway, unless you're actually obsessed with 'catching' them, any you are unable to come by you can always trade from somewhere. i haven't used it in a while so I don't know how healthy it is but I used to frequent the pokemon trading forums on Neoseeker, and used it for trading pokemon quite frequently.
If you're curious how to obtain a specific pokemon just use good old http://bulbapedia.bulbagarden.net/wiki/Main_Page (the pokemon wiki) looking up a specific pokemon there should tell you everything you could possibly need to know about it, including where to catch it in each game. (And it's easier to use and more reliable than Serebii).
To be honest, when recommending a game to someone who hasn't played in a while, perhaps since childhood, I'd actually be inclined toward gen IV or V.
I put off playing Platinum for years but when I eventually got around to it I loved every minute of it, and the improved DS graphics (compared to GBA) wowed me. However, I do think that B/W are arguably better games story and gameplay wise, so I would strongly recommend the newest generation, probably over any other.
In the end though, when choosing between Gen IV and V, there are two things to consider:
-If you're a nostalgia-fag you may prefer playing through Platinum pre-end game due to the fact there are still some old school pokemon available (such as trusty Zubat -__-)
-But in B/W while you may have to wait until after the main story to catch your old favourites, there are a lot of new and very cool pokemon (don't be intimidated by their shiny newness, I mistakenly pushed my prejudices onto gen IV for a long time, and missed out because of it) you can't catch or even trade to the old Gen IV games. And there's the fact that this gen has better graphics/story etc.
I say all this, however Sapphire still remains my all-time favourite :3
Oh, and... HG/SS are sadly overrated. As you're only going to play them for the nostalgia, or for experiencing the old gen 2 pokemon and story with a more friendly user interface and graphics, I say meh.
They were ok, but I still find I'd rather crack out the old Crystal version and play some of that than lengthen the play time on my Soul SIlver. Unlike the others (crystal, Platinum, Pokemon white) I felt no motivation to keep playing after beating the champion and and the kanto gyms. i haven't even bothered to catch the legendary dogs or Ho-oh.
But hey, that's just me.
