Life.
22 years of practice hasn’t helped.
Real answer: video games. I do nothing but play video games and yet I’m a mediocre player.
Play guitar (I can’t play bar chords)
Sing
Play poker
Tennis
Golf
Playing pool. I love everything about it (I always enjoyed geometry) but aside from the occasional bout of good luck, I’m pretty bad at it.
I do calligraphy, too. But I’m not even going to try to illustrate my letters. Oddly enough, I don’t do too bad with some concepts of drawing, like perspective, or shading. But everything still comes out looking like something an average 12YO would do.
Being in relationships with women.
Fuck.
I mean, fuck, I can’t think of anything just at the moment.
Playing softball. I’m out of shape and overweight and not terribly coordinated, but I just love the game.
Me, three. 
Stained glass. I made a lampshade that looked like a drunk person with cerebral palsy made it. No offense to drunk persons with cerebral palsy, who right now are probably making attractive lampshades. But I was the only person in my class who neither got cut with glass nor burned with a soldering iron. Take that, drunks with CP!
Baseball and board games.
Actually I don’t completely suck at board games, but my enthusiasm for them far exceeds my skill.
Carpentry.
I love building things, but no matter how simple or complex the task, or how much care and precision tools I have access to, my creations inevitably look like a forlorn cargo pallet fit only for a hobo’s burn barrel.
Karate. I’m not completely sucktastic, but it’s certainly not my forte.
Archery. I do have enough aim, at least, to hit the target, but anything after that is up to chance.
Guitar was my first thought, too… but then I realized that I actually can’t do anything well.
I suck.
Huzzah, together we’re pwned 
A mate of mine couldn’t understand whey I liked playing Halo 2 multiplayer when I was so bad at it. The idea that it was a game and fun to play, didn’t seem to enter into it for him.
I’m doing a bit of part time study, working towards some sort of qualification in computing. It’s interesting, I enjoy the exercises and that “Ah, I see!” moment. But I’m just an awful student, when it comes to the end bit, the exam, it all falls apart :o
Honestly, I can’t think of anything I don’t do fairly well, that I keep on doing. Not being able to do it well pretty much takes the joy out of it for me. On the other hand, I keep on trying to diet, which, lately I suck at! 
Pool
I played violin and viola through high school, and I was pretty good. For my senior solo I played the first movement of Lalo’s *Symphonie espagnole, *which is no mean feat for a high school student. I basically haven’t played since then (45 years). I recently bought a new instrument and have started playing again. My fingers just won’t move like they used to. Even with practice I don’t have the agility/reflexes/eye-hand coordination of an 18-year-old. And my vibrato really sucks. So playing again is both fun and frustrating.
Add me to the singers and music players. I sing whenever I can get away with it but I don’t sound good at all.
I have tried several musical instruments and can always master the basics eventually. But rarely can I get beyond that and though I can play through the entirety of a few (very) simple songs on my guitar - they don’t sound very much like music, they sound like someone trying very hard to do something very simple and only succeeding half the time.
Besides those - video games again. I play a few different types but mostly I love strategy games like Starcraft, Civilization etc (strategy board games too - hooray RISK!) and while I’m pretty OK at them, I wish I was as good as the few people against/with whom I usually play. I am of average skill at my very best. I was once informed that my love of such games would give me a dating ‘edge’ with geeky men - so far I’ve been too embarrassed to reveal it lest they discover my lack of skill 
. So much for my edge.
Another mediocre musician here. It is mystifying to me how I can actually play, say, the Maple Leaf Rag, every. single. day. for 3 months and STILL can’t get through it without stumbling somewhere. And it is never the same spot twice.
I don’t get it. But I do get that I definitely have no potential as a professional musician.
Fishing.
I wil spend hundreds of dollars to go deep sea fishing, and never catch a thing, and the guy standing next to me hauls them in one after another. I think it is the way I hold my mouth.
I just love getting out on the ocean.