Hardest skill to use properly?
Title. For me, it's either 
ult or 
goo-flip. 
grenade is a bit tricky sometimes, too.
When I say hardest, I mean either hardest mechanically, or just hardest to utilise to its full potential.
11 Comments
's Prismatic Barrier.
Your allies may be eating Blitzcrank hook's to the face left and right, getting killed with cross-the-map Ashe ults, finding a buddy for moral support and max-bounces with that Fiddlestick's Dark Wind... But by all that is good and holy: they will do the damnedest to avoid every shield you try giving them if it's the last thing they do, pulling out their once-a-day LCS level pro-play dodge mechanics to do so. :|
For me it would have to be using Vel'koz's horizontal poke efficiently and the fact that you can even proc it early. It has so much potential.
abilities like viktor E/rumble R can be tricky with smartcast on
Ult
[slayer-jinx-wink]
Beside Bard's ult, I'd say Poppy's ult.
Which is sad really. Not that its difficult, but I find it sad that with all his counterplay, his difficult to land skills and whatnot, hes been nerfed really hard.
He's mana hungry as hell, his passive stacks are only really noticable super late game, his damage output is decent but again, easy to avoid. Shame really.
The Singed Goo flip just needs set up. Easiest way to do it is putting it where you're standing, let them keep running a bit and THEN fling them. Or bush strats, easy to do top lane. But yeah, agreed in that it's up there in difficult to land skills.
My vote though? Anivia Wall. Like. GOOD Anivia walls. Anyone can wall off an udyr. But interrupting mobility spells with it takes some serious reaction time. If you don't know, placing Anivia Wall (or Trundle pillar for that matter) during a dash will break their momentum and land them flat on the ground. But seeing as how the wall has a bit of a cast time... Good luck. Also of course the wall pin strat which requires you and your enemy to be in two very specific spots.
im not sure about the hardest, but one of the hardest has to be bards ult. it can screw your team over as much as the enemy team. like if youre ulting the enemy team into the aoe abilities of your own team, youre basically winning the teamfight for them.
on the other hand if used correctly, it will have exactly the opposite effect and can completely win you a teamfight and the game. id say the difference between using it wrong and using it right is crazy, and probably much larger than most other ults in the game.
Q, with the pickup mechanic. I seem to remember someone somewhere said even Faker doesn't use it perfectly. There's a REALLY high skillcap/apm on it.
is pretty high up there too..though..it's more of her passive than any single skill of her's.
Easy to faceroll, but, if you're talking perfect reaction time/using skills AS SOON as they come back up, she has an impossible ceiling. Which is why you see more kat scripters than any other champ. Cause a bot can do it instantly.
/ 
Q, 
passive.
Again..not super hard to use, but super hard to use perfectly.
There's a video of a pro player playing Vayne on URF, it's kinda fun to watch even them struggle to Q correctly, fast enough to keep up with the cd, and still use it to AA reset and not AA cancel yourself.
https://www.youtube.com/watch?v=VBTnbV2p0Rw 1st couple minutes are boring, and he's completely fk'ing up cs'ing. But around 3:40 he fixes his shit and starts doing crazy stuff. Yes it's URF so probably not what you were asking about. But endgame pro-level Vayne in normal SR does get pretty heavily skill-intensive, URF just highlights it sooner and speeds it up a bit.