Out of the two call of duties you suggested I would get World At War since it comes with tons of custom zombies maps which you can play on. If I could suggest any call of duty then I would suggest MW1 as its still got a good player base and theres ton of mods on it which you can have great fun playing on plus the game itself is the best call of duty ever in my opinion.

Obviously Modern Warfare 2 is the correct answer. Modern Warfare 2 was the best game to come out since Mario.

My vote goes for World at War for the all the different custom zombie maps and also the multiplayer is great

Well let's take a look at all the CoD's on PC (from CoD4 up, besides BO1 because I didn't play that)

TL;DR: Get CoD 4

CoD 4 is an amazing PC Game, it's still extremely active with good anti-cheat and lots of servers, including custom made maps and game modes

Modern Warfare 2 is shit, just like the console version, hackers, glitches, lag, danger close noob tube campers, ect. All that bullshit. So that is a SURE pass

Modern Warfare 3 I loved on console, and the PC version is good too, but the game is capped at 90fps (no fixes that I found, unlike CoD4) There are a few hackers but not much. Also the lag is pretty bad (likely because of the dying community), but if you're good it shouldn't be that big a deal.

Black Ops 2 is pretty good, runs well, FoV slider FINALLY built into the game, but in terms of players, they are TERRIBLE, and if you are any good then you will likely be annoyed at having bad people on your team and having to carry all the time (as you can see here: [ Register or Signin to view external links. ] )
The gun balance is the WORST in any CoD ever made. Pistols, Shotguns, and Snipers DOMINATE this game, but obviously, if you're good you can deal with it.

Ghosts is dead, skip it.

Advanced Warfare is a shitty port I heard (Did not play)

Black Ops 3 is good. Not much to say about it though