for me it has to be 2010, the reason is because Black Ops 1 had just come out that year and the previous Call of Duty installment's were all fantastic.
The playerbase was still large
I was very young and had no grown up responsibilities i had to do, was completely free.
Xbox 360 was still at it's prime so to say, server's weren't bad and yea.
2010 for me, let me know what's yours