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.

cod4 it was less complicated back then none of these over powered killstreaks and deathstreaks

Call of Duty 4, because it was the first game that I played online.

modern warfare 2

Cod 4

Black Ops.

First CoD was CoD4 but I played that solo for the most part so eh. Black Ops, I had a party of friends and it was fun most of the time. Also, Black Ops is by far the most balanced of the recent CoDs.

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.

CoD4.

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.

Wet Work?

Bog?

Countdown?

Crash?

Ambush?

Some maps had good long range combat.

Bloc.

Pipeline
Overgrown.

Crossfire

Broadcast?

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):
Vacant

District
Downpour?

Broadcast

Most importantly:
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.

CoD3 has to my choice with vehicles, ranked or player and 24 player online.

WaW comes a close second with it's bolt action rifles and zombies.