Well I firmly believe that RE3 is the best one, for the first time in the series you could see different locations of Raccoon City, controls were top notch and you're always in suspense because you don't know when Nemesis could come and chase you. The action in the game is awesome and is only surpassed by the action in RE4. The ending was great but I won't spoil.
RE2 would be my 2nd best it has the best horror and suspense and it is the one with more replayability because it had 6 scenarios. Also it has the best story and ending!
RE4 is the most intense and has the best boss battles (Krauser is my favorite, I call him General Badass) but it was kind of easy and some enemies were simply too cheesy (zombie monks with rocket launchers and cartoony wolves made me stop taking the game seriously, I missed real zombies!). I hated that it didn't really contribute to the story of the series and also ending was lame.
Code Veronica was very story driven and the events that took place in the game were great, I liked the Ashfords and the return of Wesker (he's so cool!). The game's ending was very open wich is good and I hope RE5 has some relation with some of Code Veronica's ending elements.
RE1/REmake: I didn't really played REmake but I have the original RE, anyway what I can say from what I've seen from REmake is that it's the sacrier one, and maybe the hardest one wich are good points. From the original RE I can say that I liked it a lot it was a great introduction to the series and the opening is awesome! Controls weren't so good but since it practically invented survival horror on consoles I don't hate the game at all. The ending wasn't very good and the game was very cheesy, but afterall I keep it as a priceless gem!
Never played RE0!
