1. RE2 (1998) - hard to beat. great in every aspect. having 2 scenarios that affect each other is sooo good that i want more games doing that. extra modes are quite challenging and fun to play.
2. REmake (2002 GC) - perfect in almost every way. I'm very sad that there were no more games in that style since for me it's probably the perfect survival horror experience. I still like the original but the remake improved almost every single aspect of it.
3. RE3 (1999) - it has a bunch of problems but is probably the funnier one to play. making up challenges to finish the game gives it an arcade-like feeling. nemesis keeps being scary even when i have every place he can appear memorized

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.