For me it has to be WaW. I have always been into WWII games, having started out on Return to Castle Wolfenstein and having played CoD 1,2, and 3. I was more into WWII than modern weapons, so I didn't buy COD4 at first, as it still cost as much used as it did new. Loved the campaign on WaW, which was a tremendous challenge. The way it switched between theaters was a lot of fun. I really enjoyed the MP as well, and met a lot of excellent people and players that I still play with to this day. The Maps were large enough to allow for strategy yet small enough to keep the action going. The weapons were fun, but once everyone started using the MP40 began to display the lack of balance. I think I'm mostly nostalgic of it because of the group of friends I made playing it, and remembered being able to enjoy it and not take it too seriously.
I got COD4 months later when I began to get a little bored of WaW, and it took a while for me to warm up to it. Once I got used to the maps, I really started to enjoy it and see why it was so loved. I began to see a lot of the shenanigans that would become prevalent in later versions, and I think by the time I got into it the hackers and cheaters had already infiltrated, which tempered my feelings for it. Still the greatest Campaign storyline I've ever played, with some serious "Oh, S^$%!" moments.
By MW2 I was in the " I must have this game now" crowd, and bought it shortly after it came out. After a month, I was fed up with the hacked mess it had become. It had so many problems after release that I just stopped playing it altogether for several months. When I finally came back to it, I started playing Hardcore, reconnected with some old online friends, and began to enjoy it and improve. Loved the Campaign, MW games have the most intense storylines by far.
Black Ops, for me, is a very close second to WaW. Treyarch took all of the nonsense I hated about MW2 and helped eliminate so much of it. The maps were interactive, innovative, and infinitely replayable. I remember not dying once from a Noob Tube in the first 2 or 3 months of the game (Thats definitely changed now, unfortunately). I loved the map layout as while it did not stop camping, it made it more difficult for campers. On MW2, campers could easily hold one side of a map, but BlOps offered multiple points of entry and attack which allowed for closer , more exciting matches. Wasn't crazy about COD Points, I like earning my attachments and not just having them readily available, and loved the Emblem creation tool in spite of the multitude of "pee pees" and other childish, disgusting creations. Saw some true creativity, and every gun felt unique. The DLC maps were diverse and fun, and going back to this game I still marvel at the absolute beauty and amount of detail they put into the maps. I felt it was a step in the right direction for the series, and while not perfect I think it was the most fun, quality time I've had playing CoD. I wasn't green like I had been in WaW and enjoyed more success, and it was the first game I attained the highest prestige. Loved Combat Training and the barebones modes, and the Hardcore gameplay was right where it needed to be.
As for MW3, while it has some advancements I personally think are a step in the right direction (hearing the enemy after killing them, strike packages unique to each class, the support and Specialist kill streaks, and the advent of community playlists), will not have a special place in my memory. Its stale, there are no maps that come up that I am just crazy about, and its a step back for the series overall. Even with all the nonsense, MW2 was just more fun to play.
So for me, WaW and Black Ops are the ones I have the best memories of, and that I get nostalgic for. WaW edges out BlOps because it was my entry point. I didn't know, or care, about K/D at that time, and just plain enjoyed playing it more because of that. I lately have tried to recapture that, and just play MW3 without worrying about anything but having FUN. Man, its so not easy on this game!
