Bo2 was the one I had the best memories with. At that time I had the most friends and really had a great time playing. The other cod that is super close to that is World at War. Hands down World at War was a very fun game and I made so many memories trying to learn how to play by that was my first CoD I have owned.
The game that sparked my interest was Modern Warfare 2, I spent literally days of my life on that game. I used to make a new account on a Friday, and Prestige on the Sunday before school on the Monday! Then of course, I had my custom firmware PS3 so I started hosting modded lobbies and all that stuff. I just used to find it so much fun, whether I was playing legit or with a mod menu.
Then I got Black Ops 1 and I sweated that game like crazy, going up to Prestige 5 or so before giving up and going back to playing Modern Warfare 2 for a while. I used to find MW2 was for fun, and BO1 was for serious playing or trying to out-play one of my friends. Those were the days when gaming was far more fun and enjoyable, now it's tedious and boring.
I'd say multiple cods gave me incredible memories,
WaW for the Zombies also Bo1 had awesome zombies.
MW2 for the quickscoping was hella fun
MW3 hunting multiple moabs in games.
Bo2 league play was amazing.
I am kinda weird but honestly Ghosts. Clan Wars in that game were amazing. Every weekend my clan and I would get on. We usually had 3 teams of 6. One weekend we got into a lobby with another team so no matter what we got wins. We grinded hard for that Warcry camo & gear. Still have it to this day on PS4.
CoD4-MW2, the only ones I played campaign on, so I have memories of those.
Bo2, memories of trickshotting and continous **** talking through mics. Fun game.
Bo3, memories of being a tryhard, hit top 1k after getting the game a year late, in just less than 2 months. (Was on 360 so less players, thus it was easier to hits the leaderboard but I was real good tbh)
So many man. I remember playing the first CoD on PC back when I was like 11... I remember the names and clans and stuff we did. It was like each game had an era. I have found memories of CoD4 and MW2 as well. Kinda died after that for me.
I would have to say MW2 or BO1. MW2 because that is where I met some of my first true friends on Xbox 360 and BO1 because of the memories of staying up all night waiting for the DLC drops and just jump into zombies and hope that I could stay home from school.