Best with 5-7. 6 is surprisingly good. I would never turn down a 6p game. I would often turn down 8+p games, since I find it very hard on good guys at those numbers. With people I know, however, I'll play it at any count.
The one slight downside to playing at even numbers is the voting on missions.
In 5p, to send a mission takes 3 yes votes. In 6p, it takes four. Not a huge deal, but it makes missions go to the Hammer slightly more than at 5p.