For me it’s

1. Resident Evil 1 (1996)

2. Resident Evil 2 (1998)

3. 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.