[COD] Wich one should i buy???

I'm new to the Call of Duty franchise and I'm really keen to try some of them out. I'm not really interested in online gaming, I'm more into the campaigns. And I've heard a lot about them. I know that the MW series (at least up to 3) is good. Then I've heard mixed things about Black Ops. Some people have told me they're very good, and others have said they're very bad. I've heard mixed things about Cold War. And then there's the problem that some of the games are very large, like 200GB. Which ones do you recommend?

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