I would say World at War gave me the most fun. I felt my skills grow the most while playing that game mainly because most of the guns took a respectable degree of skill to use. The semi-auto ARs, the bolt action rifles, the recoil on the guns, etc. I didnt really like the MP40 but many believe it was overpowered but other than that it was a very balanced game when it came to the weapons. I think WaW had some of the best maps too. Awesome maps for sniping and LMG usage and great maps catered towards shotgun and SMG usage as well. I felt the tanks were a nice twist to the mp aspect of CoD but they frustrated a lot of people. Regardless I think I enjoyed WaW the most because I raged at it far less than I did with other CoDs.
Modern Warfare 3 is actually the most fun and favorite for me. I felt I have a great degree of freedom on what I can/choose to do within the system.
MW2 and WAW are not far from MW3, but I'd play MW3 over the others.
However cheaters ruined this great game. Lagnet wouldn't be an issue if kids wouldn't throttle their bandwidth in order to gain advantage. Imagine a 4 bar got beaten by 2 bar just because the 2 bar player limited the upload speed.
As much as it pains me to say this MW3 has been the most fun for me. Here is why.
I was late to the FPS online multiplayer party. I picked cod4 mw2 and black ops at the same time it was about 6 months post release on Black ops. I only did campaign on cod4. Played about 10 games on mw2 before I was lobby hacked. I was flat out terrible in Black Ops. Just about the time I started to figure out how to play a bit better mw3 came out.
I struggled with the game for the first month or two then it started to click for me. I finally got my play to what most on this board would call average which for me is way ahead of where I was in Black ops. Add to that putting together a clan of 9 guys that play well together and it has been quite a bit more fun for me than the hacks on mw2 and the beatdowns I was taking in Black Ops
COD4 was the most fun for me. It was my first FPS multiplayer experience. I knew nothing about overpowered this, cheap that. I didn't know any of the things I know now to complain about. Ignorance was truly bliss then. I played the game without caring about dying and had no expectactions. It was a blast having no expectactions. It has become less fun the better I get at the game, as I come to expect more and also know every little thing that is out of place.
mw2 and cod 4 i loved the multiplayer of both of them. single player is wow because me and my brother did it together, now that was fun besides all the bloody granades lol.
i have never enjoyed single player, but doing the campaign with fam or friends is amazing.
lag ops was boring for me. not sure why, i think maybe it was the kill streaks.
and now mw3 is just to laggy. what ever they did in lag ops and mw3 to the connection has ruinned cod for me.
but i will always remember all the fun i had in the other cods.
ALL weapon balance problems:
M16 slightly underpowered
G36C should have more range than M4.
R700 slightly underpowered.
Maps were uncomplicated, some of them based on real locations, and there wasn't clutter all over all the places.
They had a sense of direction, as the enemy spawned on the opposite side of your spawn, with three main roads to take to come to you instead of 5+.
(Yes, most maps in MW3 have 3 main roads, but they are not where enemies are.
Enemies are where there's 5 roads each leading to a hotspot where you'll die if you go into the open, all the while you have to enter that hot-spot if you want to find the enemy.)
Some maps were unique, open and were challenging without being mazes.
Some maps had good long range combat.
Some maps were uncomplicated (nothing like a maze-layout; only two enterances to each hot-spot and combat is mostly on one level AND less random):
MOST of the maps were rectangular, and had spawns on and near the ends, giving them direction.
I miss not constantly being shot by someone I killed 2 seconds ago, getting double and tripple killcamss
I miss KNOWING that there aren't enemies behind me when I spawn, and knowing that there certainly aren't enemies behind me when there's teammates behind me.
World at War was probably my favorite and most enjoyable game. Even above CoD4. I really liked the weapons, map design, and even kill streak set-up. War was an amazing game mode, and tanks were a nice change of pace because it encouraged teamwork.
The addition of Nazi Zombies was also great, because it provided a relaxing alternative to competitive MP that you could play with friends. IW has tried to replicate it with their Spec Ops and Wave Survival modes, but it simply hasn't caught on like Zombies did.