Good day to y'all. I'll explain my situation. I first played the old Game Boy Pokemon years ago (so many years ago) and I liked them. But I ended up growing tired of them, so I didn't ever touch the pokemon series again. But now, I think I may replay them again and get me some new game, but first I'd like to know what are the differences between games? All those editions... Do they share the same plot? The same gameplay? I can't remember the old red/blue/pikachu edition, but I think the only thing that differed were the pokemon you were able to get. How does it work now? And what about old and new pokemon? I heard there are now a lot more than before (about 700, I've heard, but don't know if that's right). Are we able to catch all of them in a single game? I don't think I'll get these new X/Y editions, tbh (I dislike the 3D thing), but I'd like to know... If I get, let's say, Black Edition, can I still get all 151 original pokemon while still having all new pokemon available?
Anyway, I think I'm not explained it very clear, but I hope you understand. Thanks in advance.
In your case if you simply want to replay the games of your childhood memories I would get FireRed or LeafGreen. Those two games are remakes of Red, Blue, & Yellow.
I say this because many fans of Generation 1 don't like the addition of so many Pokemon and all of it's new mechanics. We call these people "Genwunners." If you aren't a genwunner, then by all means purchase Black 2 or White 2. If you think you're more towards the Gen 1 side, I'd just get the remakes. (Even though they are still pretty old. Released in 2004-2005 I believe.)