It has to be Black OPS
I choose Black OPS as i have played MW1(Yes Call of Duty 4) MW2 and Black Ops and by far Black Ops has tha most amazing campaign you will ever play. I was astonished as i thought MW2 storyline was good. If you get the chance to buy Black Ops i highly recommend it. Also the multiplayer is great as there is no Stopping Power or Juggernaut - most annoying perks. It makes the game fair and fun. Also Nazi Zombies is back from World At War. It much harder and has new types of zombies.
To Conclude Call of Duty : Black Ops is the best game in the Call of Duty Series.....Without a doubt...
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For multiplayer, Call of Duty Black Ops was almost perfectly balanced, Call of Duty 4 came close.
Modern Warfare 2 was completely broken, it was a lazy-made console port and as for Black Ops; it has great potential but Treyarch has somehow managed to mess up the netcode for the PC-version. It is not clearly connecting at all and lags continuously of course patched later.
As for singleplayer, Black Ops, Call of Duty 4 (there is no such thing as "Modern Warfare 1") and Modern Warfare 2 did fine.
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In singleplayer Black Ops has a great campaign plot, but it rips off some stuff from MW2 and has a worse online multiplayer setting. So I'd say either MW2 or Black Ops
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I thought Black Ops had a pretty good campaign. The story isn't as straightforward as the other campaigns in the COD franchise, and even though the plot twist was predictable, at least it had a plot twist (Who cares about Shepherd? I see him nothing more than a device to progress their the story than an actual character). Black Ops is pretty in-depth in terms of plot, in my opinion.