For me it's
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Resident Evil 1 (1996)
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Resident Evil 2 (1998)
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Resident Evil Outbreak (File 1 and 2 because it's pretty much one game split into two)
1 will always hold a special place in my heart since it's the first I played. 2 because it improved on everything that 1 did and it was the first Biohazard game I played to completion. Outbreak because it took the series online and introduced multiplayer while still sticking true to it's survival horror themes.
I treat the three eras of Resident Evil as their own separate entities, but if I had to pick one from each
Classic Era: RE2 (1998). REmake is a great game and probably the pinnacle of classic survival horror in many ways but the story and characters really let it down for me. RE1/Remake is like some straight-to-video Italian zombie flick while RE2 is an 80s James Cameron movie
Middle Era: Resident Evil 4. The first of the action oriented Resident Evils. Self-aware, campy and most crucially of all it was still an uncompromised single-player game
Modern Era: I'd like to give it to RE7 since it was a solid, original entry that revitalised the franchise after the huge stinker that was RE6, but the RE2 remake is a better put together game overall even if half of what makes it good was just recycled from the 1998 original
I want to play that, feels like it gets overlooked now thanks to the remakes and village.