A free and good one seems to be World of AI. http://www.world-of-ai.com/
Payware comes with Ultimate Traffic 2, My Traffic X, My Traffic 2010, Traffic X. Don't mix them up!  :P
They are all ok I think, preferences and prizes may decide about the best buy there.

The FSX default traffics eats up quite some resources and also has only few models, you will see real crowded skies with any addon there.