Complete the first one. The story is actually interesting enough this time 'round. The sequels have whole different areas, but there's a lot of same-y-ness to it still, so it depends how much you want another Pokemon.
The Pokemon World Tournament is pretty good though. As are the Funfest Missions and the Medal system. They add a lot to make the game seem a little fresher. --- Game news is the best news.
I played Black 2 without playing Black or White. Took me a week to beat the Elite 4, catch post-game legendary pokemon and explore the previous inaccessible areas.
I understood Black 2's plot pretty well as the in-game characters recap whatever happened in Black/White.. but I am tempted to buy Black and play it to see how different it is from Black 2 (Gym leaders, Elite 4, Legendary Pokemon.. etc) --- Currently Playing: Pokemon Black 2
Basically - Black 2/White 2 are to Black/White as the second half of Gold/Silver was to Red/Blue.
You're going through the same area, but a couple years have passed and some things are different (a few new gyms, people shuffed about, some changes to the landscape, etc). For newcomers it won't matter, but having played the first game(s) add some flavour and nostalgia to things. --- "Don't go. The drones need you. They look up to you."