Offline - has to be Halo, so many consistently good stories and memorable moments.
Online - has to be Call of Duty 4 for me - I put ridiculous amounts of hours into this with friends. We'd end up in a ranked lobby for hours at a time. There has been nothing like it since. The closest I can say was Titanfall 1&2 which were so smooth to play. They are the only 3 FPS I put any amount of time into really, I get so bored with anything else.
think I clocked up around 3 weeks worth of playing time on COD 4, my friends had more than that. Just incredible to me that I spent the best part of a month of my life playing that game.
think COD 4 got an extended life purely because many people didnt like the idea of going back to WW2 in WAW so just stuck with modern warfare for another year.
I still think its a total joke that activision thought it was perfectly ok to charge the same amount for the map pack in the remastered edition despite the game being a decade old plus adding in micro transactions. Money grabbing bunch of bastards.