There's not one genre I find particularly hard. I mean, some FPS's are so hard they make you want to claw your own eyes out, but others are so easy you could play them with one hand - same goes for RTS's and RPG's.
Easiest game I've ever played? Probably Tekken 5. Only thing that comes to mind right now where I never died/lost even once. (But I've been playing the Tekken series since I was five so I knew character move sets off by heart.)
Fallout 3 was also pretty easy. On the hardest difficulty, the only thing that came close to being a threat was Feral Ghoul Reavers, because they could take so much damage that my weapons would degrade or run out of ammo before I killed them, and even then I don't think I ever died to one.
Hardest game I've ever played? I don't know. Possibly old school Prince of Persia, but I was playing that when I was just a little kid, which makes a difference. The only other one that I can think of is Far Cry 2, not because I died a lot, necessarily, but because the first time I played it (on the hardest setting), the NPCs could absorb so many bullets that I wasn't even sure I was hitting them.