I've absolutely loved Left 4 Dead. Before it I mostly shied away from multiplayer but it's engaging and fun co-op has finally gotten me properly into multiplayer, even if I'm a bit shite..
As for Resident Evil, it's deviated away from zombies in recent iterations. Resi 1 to 4 were pretty good, with the occasional niggle insofar as difficulty went, but Resi 5 is pure, unbridled hell from chapter 4 onwards. My GOD I hate that game. It lulls you into a false sense of "Oooh, this is quite good! Fun, even! And co-op!" before throwing you head first into the most ridiculous, bland, irritating levels and boss fights imaginable.