For me, it's two things: green beans and brownies.
I can't seem to find a happy medium with either. When I cook green beans, they always come out way undercooked and almost raw, or mushy and overcooked. I can't find that happy place where the beans still have a bit of crunch, but aren't raw.
Same with brownies. I either overcook or undercook. They'll come out raw in the middle, or so cooked that the edges are inedible. I have no problems with cakes or breads...just brownies.
So, what is your cooking nemesis? That thing you just can't get right no matter how many times (and different ways) you try?