For me, for whatever reason, as much as I want to use Rek'Sai and Kha'Zix, due to my love of the Void in general, and insect type characters, I will ALWAYS feed and get stomped super hard whenever I use either of these champs. It doesn't seem to matter how much I practice, what I build, I just can't use them, and it makes me sad. Share the misery, who do you love, for looks, mechanics, whatever, but suck with?
I think I've had maybe one game where I didn't feed in lane and that was because I could blend in with 
's tormented soil
has been nerfed to the point that she must build full tank to do anything, and only coordinated teams like in the LCS can really take advantage of the map presence she brings to the team. Not your fault you can't make her work, she's my favorite jungler but I stopped playing her because it's so unsatisfying being the knockup bot they turned her into.
But as far as the question goes, 
. I try to harass with Q and I push the wave and get ganked. I play it safe in lane then I get peeled in teamfights. I try to roam and people see me coming and casually walk away. I just can't make him work.
Sarcasm Incoming I am good at every champion and therefore can play anyone that I want at the skill level of LCS players
DRAAAAAAAVEN
Misses axe "Okay, fine. I'll spin another one." Misses new axe on the first bounce "...i wanna play Jinx again."
I've got the best skin for him too, I'm just bad at CSing/comboing with him.
, He was my first champion ever purchased, sadly his rework hit shortly after I had started League and I've fed every match I've played as him since then. I'm practicing with him in bots currently to rid myself of frustrations from a bad losing streak.
Twisted fate! Not sure why but I just can't pull him off. I don't feed when I play him, but I just do terrible in terms of getting kills, CS, or doing enough damage! Although I do remember playing him in URF granted as an adc and it was really fun, but for serious games I just can't pull him off.
I just can't play as them and I don't know why
I can't jungle for squat so.....yeah, kind of in the same boat.
My first ADC, literally my first, I owned him before I owned Ashe or Sivir. ADC being a role I suck unless I'm playing Tristana, makes me want to cry when I see this champion being played really well because why can't I do that?
His rework made him unplayable for me because I suck at comboing his skills properly.
Legit don't know how to use this guy at all. I like everything about him but I just suck.
(just suck with him after a few games) 
(competent with TF but every time i play him i feed hard XD) 
(tried and failed miserably) 
