What is the best pokemon to defeat the elite 4 in pokemon black and white?

There is no one right answer for this because the elite 4 has a variety of pokemon types for you to challenge.

One tip I can give you is that your starter Pokemon is an invaluable partner if you've raised it through out the entire game. It should be very powerful right now but try to get it to as high a level as possible and that way if your other pokemon get into trouble your friend can back you up. Also give it a variety of attack types so you're prepared for multiple typings or even if you only have a few types try having one for special attack and one for physical.

Having multiple types of pokemon is very useful as well. If you find you have a lot of the same typing try catching something completely opposite of that typing to balance it out. If you have lots of types you'll have much more flexibility and more options to use against the elite 4 upping your chances of winning.

If you could let me know what kind of team you have at the moment I could help you further and give you specific Pokemon that may be useful for you individually to catch.

I actually defeated the Unova League with only one pokemon on my Black 2 game and it was Genesect so I think that might be the best choice. The only problem I ran into was with Shauntal's Chandelure, but apart from that it worked out.