Which would it be?
Call of Duty 4, MW2, and Black Ops 2 are the best CoD games in my opinion. If I had to choose one, it'd be hard because each of these games represent a golden age of Call of Duty. CoD4 built the foundation that CoD is still based on today. MW2 was such a huge game changer and popular game that everyone and their dog's puppies had it. Black Ops 2 was the last "great" CoD in my opinion. So I would have to choose one of these three. My final pick would probably be the original Call of Duty 4. One, if not my all time favorite video game. I think the remaster of it, MWR, tarnished its reputation however. Too much DLC packed into supply drops, especially weapons.
That's my main gripe about most video games nowadays anyway. You can't just buy an online game anymore and earn all the content, from cosmetics to weapons, by gameplay only. At least some of it can be unlocked by both completing challenges AND microtransactions.