Mostly all of them are good in their own way, but...
The remake of the first one for the Gamecube is incredible, probably the best one, and it's actually (dare I say it)... scary. Yeah, Resident Evil, and scary isn't really paired together all that much. It's graphics still look bloody gorgeous too, even though it came out ten years ago.
Resident Evil 3 is a great game also, I just love how the Nemesis pops up everywhere, even when you least expect him to. God I love the Nemesis! Best enemy ever.
The Dreamcast version of Resident Evil 3 is the definitive version, and has the instant unlockables and smoother graphics.
Most of the spinoff's are pretty rubbish, with the exception of Outbreak, which is pretty awesome, but has some of the longest load times I've ever experienced in a game.