Tough question . Putting graphics asside , i'll go with RE 2 . It pretty much nails the aspects of survival horror in my opinion . Back then the game was being marketed as survival horror and that's exacly what it delivered . A lot of the mechanics will seem dated by today's standards though . Limited saves , limited ammo , limited health items , limited item capacity , no checkpoints, very real threat of death . As for horror we got , diverse enemies , quick enemies , bullet sponge enemies , unkillable enemies , jump scares , creepy atmosphere, creepy music, feeling of being traped in a hostile environment , gore .
As for the actual game . We got a nice 10 hour game on your first play through , with two characters , each with two scenerios . Then we got 2 secret characters (hunk/tofu ). Plus you can best the game in under 3 hours if you know what your doing . The game is a true classic to the genre .
Resident evil 4 is a close second . It went a more actiony route , while keeping survival and horror elements . It's the perfect blend of two worlds . It brought about a nice change of pace. Broadening it's horrizons for those who like action games and those who like survival/horror games .
The problem with RE5/6 in my opinion is that it dropped it's survival/horror fans and focused soley on the action demographics . Which is perfect for them , but not so much for the survival horror demographic . But i cannot blame capcom for that , survival horror is a genre that is dead and VERY niche . It kinda went out of style .
So basically , the game went from survival/horror(RE1-3) , to action survival/horror ( RE4) to action/horror ( RE5-6) . So a lot of people wouldn't touch the first three and another batch of people won't touch the last two .