Growing up all my friends were obsessed with both San Andreas and Modern Warfare, so that's a pretty good match up. Personally I've never understood the appeal of GTA, sure it was exciting as a kid to run over hookers and kill people with a baseball bat, but as an adult, now my eyes glaze over when I consider playing yet another crime sandbox. Everyone's sick of CoD, but I think Modern Warfare is genuinely great. If someone were to force me to play either game until I finish it, definitely would go with Modern Warfare.San Andreas is beating Modern Warfare, Witcher 3 is beating God of War, GTA5 is beating Arkham City, Breath of the Wild is beating Bloodborne.
Witcher 3 and God of War... that's hard. God of War is such a high quality and curated experience. Witcher 3 is a lot less focused, but there's just so much more of it that is also high quality. I think God of War is more fun to play, but Witcher 3 was an experience that will stick with me for a long time. I really can't choose.
Arkham City is one of my favorite games of all time, although I am a huge Batman fan so definitely biased. GTA V is just so huge though (both in terms of content and cultural impact) that even I don't give a shit about it, I can't quite put City over it.