You should get Black or White first before getting Black 2 or White 2 cos that way you can prepare for the B2/W2 story by doing B/W first. As Cyclone said, if you play Black/White first cos it gives you a better idea of the Unova story.
Also, having Black or White can give you an extra feature in B2/W2 called Memory Link, where you can connect your Black 2/White 2 to your Black/White and see some cutscenes that can't be seen without it. There's also one-off post-game battles against Bianca and Cheren in B2/W2 if you use Memory Link. There's lots of good features in B2/W2 you can only unlock if you use Memory Link.
And if you have Black and White 2 or White and Black 2, you can get both Reshiram and Zekrom if you trade one of them over.
I'm adding on to Forever's post and saying you should get Black and/or White before starting on Black 2 and/or White 2.