I'd say most competitive is 8pm - 11pm, I like playing in the morning 6-8am, easy mode for the most part. (For Black Ops, which is pretty much all I play online)
I would have to say 9pm - 11/12am during the week. Weekends depending on the day or how busy I am and what not probably any time I can. I do got other things to do besides gaming, but I do enjoy it very much over all. xD