This is a very tough one for me to be honest. I have to say sports games or racing simulators. I just get bored to easy by those games and don't really care to play them. Though that hardly has anything to do with difficulty, but rather personal preference. Hmmm I'm ok at RTS games but I do lose more than I win so that might be my weakest genre on a competitive level. While I am quite good a fighting games, I tend to be good at games with a block button or counter abilities. So like Soul Calibre and Dead or Alive, I'm not so good when you need hold back on the analog stick or d-pad like Street Fighter or Mortal Kombat. I just can't do it very well for some reason and naturally try to block with a button or dodge down.... So while I am good at some fight games, not so much for the back block ones. I would still say RTS is probably my weakest (if sports games can't count).

Fighting games for me personally.

JRPG's or strategy for me. FPS can be fun. There's other genres that are difficult for me but I don't 'em.

SHMUP no doubt. I love those games a lot but man are they difficult, some are bit more manageable like Life Force but I can't even imagine trying to do a no credits run of a game like Ikaruga.

First person games. Specifically shooters or play formers.

Probably FPSs. I love the genre, but I do suck at it

I adapt to all genres... some moreso than others. BUT! The one genre I can never improve on is racing/driving games. I'm beyond terrible at them.

Fighting games, I just can't do them at all, so so hard.

RTS, competitive play only though(hate getting pressured), single player or coop is fine.

I'm pretty good at it all. I find that people who find any genre innately "difficult" have simply never put in any real effort. I've never had any difficulty with a genre that I've put time and patience into, which is all the ones I've tried.