5th Gen I've never played 5th gen. Where should I start?

TL;DR: My ds broke once 5th gen came out. Never got to playing it really. I'm just wondering if I should start at bw2 or bw1 and what are the benefits from one and the other.

B/W2 has more Pokémon, and thus easier to make a stronger team, but as MissDreavus said, it won't make much sense. Also, B/W was before that 'We'll merge Zekrom/Reshiram with Kyurem to create a hideous, bad designed monstrosity' idea. =) Definitely get B/W before you get B/W2, but I have one bit of advice for B/W.

DO NOT UNDERESTIMATE BASCULINS POWER!

As everyone else said, I recommend starting from B/W. First of all, the games' storyline actually begins there.
Second thing, there are only Gen V Pokémon in B/W. You can't catch any Pokémon from Gen I-IV before you beat the Pokémon League.
What this means is, you'll have a lot of time to get used to the new Pokémon and learn their types, moves, etc. so I suggest you play that and get to know Gen V Pokémon better before playing B2/W2, which features Pokémon from previous generations along with the newer ones.
Last thing, if you do end up liking B/W, you can - as someone already mentioned earlier - use the Memory Link function. It's very neat, really, and helps you understand what happened between the original games and the sequels. This is a pretty minor detail but I thought it's a very nice addition to the storyline. Of course you can just ignore it if you don't really care about this type of stuff.

If you wanna say "oh screw the plot, I just want a fun game" then you might want to consider B2/W2. It's not as storyline-heavy as the previous games, has a lot more fun stuff, a richer post-game experience, and new gimmicks you won't find in the originals.

If I was you, I'd play B/W first so you can get an idea on how the Unova story starts out since this generation was very story-centric. Also, as mentioned the Unova Dex only has Unova Pokemon and this can help you get used to them, especially considering that B2/W2 has older generation Pokemon added into the Unova Dex.

And having both B/W and B2/W2 has another benefit, and that's in B2/W2's Memory Link where if you have a second DS/3DS system (and I mean you need a second system as the wi-fi got nuked almost a year ago) and it'll acknowledge your journey from the original B/W as well as bring the story between games together. You don't have to use it, like what I did when I played my first run of Black 2 (which sadly corrupted its save file so I never play it anymore).

I know some people only prefer to play one of the games but having both on hand has more benefits as far I know. Good luck and enjoy the game!

Its pretty much a no brainer. its way better off to start on BW1 since that's basically where much of the story of the Unova region is. Also, in relation to what's been mentioned already, starting off in BW1 where no previous gen pokemon can be caught until the post-game means that you'll have more time to appreciate what Gen 5 offered. Not to mention, some pokemon in BW1 have learnsets that are downright ridiculous such as Excadrill learning earthquake at lvl 33, so there's much earlier power spikes.

As every1 suggested, you should start with BW 1..
Btw, I LOVED gen 5 games... made me return to the Pokemon world after years of being absent :)

Everybody else has already said what I would, but if you only have the capability to get one game then B2W2 would probably be your best pick. That's not to say to pass on BW, they're definitely worth your time to play, especially if you're someone who likes Pokémon games for their story.

Get black and white 1 first . You can do memory link as stated above and you can battle Bianca/Cheren in black and white 2 as well and there are also special pokemon that you can get in the story when you do memory link .