Runescape...definitely Runescape. I have 3 99'ed levels on there ahahaha. SOOO MANY hours.
But 2nd would be MW2 on the 360. Loved playing that with my friends back in the day.
I used to love Runescape, until Jagex...
I also used to love Modern Warfare 2, best Call of Duty game in my opinion. They should remaster Modern Warfare 2, and fix their servers and such. All they needed to do was implement dedicated servers so Host Migration wouldn't pop up. Plus there's tons of hackers now on Modern Warfare 2.
Good times though, those games will be remembered