It's a very personal choice... do a search of the forums ('favourite maps' or something) i am sure there have been about a thousand threads on this- newest one is under: Post your top 5 maps here!!!
Choose maps you are good in, and if you communicate well with your partner play on maps where you can easily work together and fort to each other, etc. I actually like Doodle Earth and CCU, even though I'm not great at either of them.
Give bigger maps a go, they allow you to do more as a team and generally its less dependant on luck that way.