Hmm, can't really decide, so I'll just make a top 3 if you don't mind.
1. Resident Evil 4
2. Resident Evil 2
3. Resident Evil 3
I like RE4 best because of Leon S. Kennedy who is my absolutely favorite character.
And then I liked RE3 next most untill I discovered the cool final bosses in RE2, and the fact that the final boss isn't the final boss after all. You think that it is the mutated form of Mr X (anyway I have read that is his name), but then when you think you are safe, William Birkin again tries to stop you. I just love that surprise!
RE3 is mostly because of Nemesis, who I also think is pretty cool. And I also think that the explotion last in the game i very cool, the way the wave of fire just blow the city away like nothing.