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 think it was just a good horror game in general tbh.
The atmosphere of being in this large creepy house on the bayou with this weird family was neat and there are moments that are actually kind of suspenseful.
I don't have much experience with the games outside of a group of youtubers playing 7, Village and the remakes tho.
Now's a great time to get into the series since all the mainline games are so easily accessible via emulation or have PC ports. 1,2,3 are all on and coming to GOG.