Given I've got good 5th Gen knowledge I should probably give recommendation that you play Black and White first before playing Black 2 and White 2. The reason is playing the original pair before the sequels is that it will give you a better explanation of the Unova story and the two years between B/W and B2/W2.
Also, Black 2 and White 2 have a special feature called Memory Link, which is when you connect Black 2 or White 2 to a copy of the original Black or White. It unlocks special cutscenes that aren't present without it, the ability to catch the Pokemon that N used in the original Black and White, and it's basically a cool feature in general.
So yeah, the conclusion I have come to is play Black and White before Black 2 and White 2. You can experience the whole Unova story that way, and if you only get like say, Black and Black 2 (or White and White 2) you can get both Reshiram and Zekrom that way.
We've had these questions a lot (with the latest thread on this expiring just yesterday!) Because of this, I've added a section to the FAQ to cover this (which you can see in the stickies of this section). However imma lock this up because you've already recieved a few responses and we get these questions so often. So from now on this is the last thread of this type, and in the future we'll direct people to the FAQ. Hopefully you decide to get B/W ;)