1-2-3 dont even come close to 4-5. 1 i played for 30 mins and got bored, story is pretty realistic but gameplaywise i could not keep up.
2 is alright, pretty liniar for my taste, stealth wasn't really an option,
3 i enjoyed had some solid weapon customazation, and had the best dlc pack in any game ever (the Camouflage Weapons Pack) this dlc made the game, sniping is one of the best in the series, stealth gameplay for the first time was a viable option but had to improve,
4 was a big jump, where things really started out to be great. 4 had way better maps and better character roster, although i like the ghillie suit and the airborne elite skin in 5, everything in 5 from weapon customization to stealth gameplay is more enjoyable. storywise its even for me, i know the story of both games but i dont really care about them, it nice you have a goal in the game but both stories are good enough. stealth gameplay is much better in 5, the adding of audible range is both realistic and really cool to experiment with, same with the added ammo types.
In short, 4 had better maps, but 5 is the winner for me because of the excellent stealth gameplay and extensive weapon customization