Can't say I've played all the resident evil games, but the worst is Operation Racoon City. I'm not really the type of person to like that gears of war play style in a game, the Resident Evil series brought the challenge of not being able to reload and shoot
which was fun and really classified the noobs from the professionals just by looking at how they approach this. Didn't mind Resident Evil 6's control system as much, it seemed to work a lot better than the one in Operation Racoon City.
Best had to be Resident Evil 5. Just slightly under this was Resident Evil 4 because I'm a huge mercenaries fan, but it didn't have any real co-op features. Survival mode is practically my favorite mode in any game so being able to play it with others is
really the only reason it is above Resident Evil 4.
I'm going to agree with A Dumb Toaster, but switch 5 & 4. 4 is my favorite because I always felt like every enemy was overwhelming when I played through it the first time. The co-op aspect left me without the survival feel, but it's still a great co-op game
imo. Operation Raccoon City is one of the worst games ever made, 6 following a close second.
*Accidentally put "not being able to reload and shoot" but meant not being able to move and shoot in the original post. (just to clarify)
Also 4 is the only Resident Evil game to actually offer some tricky puzzles, one of the reasons that I hated because it sometimes slowed gameplay but made the game more fun having to deal with zombies while multitasking to figure out the puzzles. Chapter 4
was the only chapter to involve some type of puzzle in Resident Evil 5, but honestly having to focus those sun beams was a piece of cake, it took <5 minutes max first time figuring it out. Operation Racoon City or Resident Evil 6 hasn't included any puzzles
from the campaigns I played before I quit playing it, but it didn't look like anything challenging was prevalent.
Like you mentioned, the game was a lot more overwhelming. On my first play through of 4 I remember killing the first few enemies thinking I was awesome and dying in the village past the rock quick time event multiple times thinking this game was too hard or
there was a way to skip the entire fight. Now part of the fact might have been me unknowingly triggering the chainsaw guy by entering every building in search of ammo (I had terrible aim the first time playing and ammo seemed more valuable in 4), but every
boss and enemy seemed more realistic when attacking, they had no remorse. In 5 whenever I walk near a chainsaw majini they will yell for about 2 seconds before actually having the balls to hack off my head, in 4 I don't even attempt trying to walk around a
chainsaw guy because they just swing without a yelling animation and it is an instant death. Even the reapers will do a short scream before grabbing in 5, and they're one of the enemies that are intended to be feared.
Had Resident evil 4 been co-op when they ported it to the 360 or actually made new modes or something it would probably been as good if not better than 5, but I can only play a single player game so much until it gets boring (I barely touched RE4 on Xbox360).
Co-op in 5 did make it less scary over replaying it, but when I first got the game I didn't have Live so it seemed almost as challenging as 4, but I expected to fight a huge horde of enemies at the start of the game this time.