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.
honorable mention to outbreak. i only played it in single player mode but i'm sure it would probably enter the list if i had the chance to play multiplayer. between it and REmake were two very good ways of moving the franchise forward and keeping the formula interesting. sadly, they went the action route so we got okay-ish games (4 is great although it still ranks way below the classics for me) instead of hypotetical masterpieces.
Oh yeah multiplayer was where it's at, never to late to try it out though as fans have kept the game alive via private servers you can easily connect pcsx2 up to.