For me it's gotta be the original; 2 and 3 were more streamlined but didn't offer as much tactical variety or interactivity with the world.
The sequels aren't bad games for what they are, but they don't stand out from the crowd as much either, being well made but ultimately rather formulaic AAA shooters whereas the original had its own unique flavour.
Bet with Liquidlaser: I say PS5 and Xbox Series will sell more than 56 million combined by the end of 2023. (And over 130 million lifetime)