What champion whose theme you love can't you play due to kit wonkiness or insane skill ceiling?
For me it has to be 
Aurelion Sol. When this champion came out I was beyond hyped and his lore, VO, and abilities on paper all made me super excited. A roaming constant damage mage who builds a little tanky and shreds with positioning? Hell yes. I played a ton of him, got him to level 4 in the matter of a few weeks and was going for the coveted level 7 when the nerfs started to strike. Let's face it, Aurelion was super OP on release. Right now I'd say he is underpowered but has a small subsection of mains who are so insane with him that he can't be buffed for the common man, and why would those people want him to get "power neutral changes," just look at what happened to Kindred, Taliyah, and Poppy. Sure, they have better winrates at the bottom but now they're useless in high level play (with the exception of jungle Taliyah... which Riot will probably stop if it's too extreme.)
Aurelion's character and kit is just so cool to me but his balance feels so unsatisfying. Constant damage based on your positioning sounds badass except early game you do about 3 damage and are still mana gated like hell, then late game your minimal damage is upgraded to a pretty alright shred except you'll die instantly in the lawnmower instagib teamfight deathball meta.
Some other characters for me include 
(whose old kit I loved but the season 5 rework isn't as fun to me), 
half of his skill ceiling is explaining to your teammates how your kit works and getting them to not jump out of your belly.
He just feels so awkward and strange in how he scales and itemises. Every build feels like you're really improvising to make him do things that other champions do with less hassle. Hell, even Urgot's itemisation feels better and he's cut off from half the items he wants since he's ranged.
and 
