Resident Evil 4 - must have. Last really good RE game, a bitter sweet one, as it kick franchise into actionland.
Resident Evil 5 - weaker than RE4 in almost every way, but still fun to play, especially with coop partner. Don't expect survival horror though.
Resident Evil 6 - the strange one. 4 campaigns, 4 main coop duos, 4 main protagonists. Every campaign suppose to give you different feeling. Leon and Helena should give you RE2 vibe, cause zombies are back, and gameplay takes place in small American city infested by them. Sherry and Jake should give you RE3 vibe with unkillable mutant constantly chasing you. Piers nad Chris should give you a RE5 vibe with focus on action. Ada should give you vibe of original RE with focus on solo gameplay and riddles. Of course game didn't deliver on any field. It is still decent game fun to play especially in coop.
Resident Evil Revelations - quite fun experiment with decent gameplay and poor graphic. Worth a shot.
Franchise is struggling lately, and finally even Capcom seems to understand that. We have an Revelations 2 and HD re-remake of original RE. Hope it sell well, hope it will influence hypothetical RE7/reboot.
Capcom need to do what we all want, Leon and Claire in SH focused main Resident Evil game.