I agree with a lot of these guys, mostly Rumble and Aniv. I'm actually pretty solid with Anivia but her positioning is awkward with the short range ult and she's squish, plus her lane phase is rough if you don't know how to farm and use her good range. Rumble, however, for whatever reason, I can't use well; I've been meaning to try him again!!

I think the HARDEST champion to play would be....

Malphite

The most difficult champion I have bought and played is Ezreal (In My Own Experience!) I have really gotten a hang of his skillshots though and one-shot sniping people with my Q when they think they are safe under tower with ~80 HP is just funny. Dat range.

Edit: Forgot to mention Anivia! She has quite the skillcap. Setting up your wall in the most beneficial position, landing that Q just perfect so it stuns your enemy, she has a lot of difficult mechanics. When I lane against Anivia I usually have an easy time dodging her skill shot as it moves maybe slower than Morgana's Dark Binding. I played her in free week and you have to be dedicated to that champ to play it well, not like Graves where you pick up and play.

this question varies from person to person I for example can not play ad trydemere at all

Twitch, Orianna Twisted Fate.

Twitch because of his Ultimate. It's fairly hard to learn how to aim but it's worth it in the end.
Orianna because of her very high skill floor, and her deviation from other champs. I've been playing her this week and I'm beginning to like her. Very interesting, to say the least. She's also got a lot to manage.
Twisted Fate because of good use of Destiny, Gate, and when/how to use Pick a Card, something that would seem like it doesn't make a huge difference, but really, really does.

The hardest champ that I've tried to play would have to be Anivia. Tried her one free week, and just struggled to really use her effectively. Mostly because my wall positioning was a cry in shame, and unless your lane opponent derps hard, it can be difficult to land her stun. I also found Talon rather difficult to play effectively, but I think last played him when I was new to the game, so that might have contributed to that perception.

I'm personally surprised to see Rumble mentioned so often - I tried him out last free week, and absolutely dominated with him. The only lanes I had trouble against were solo-laning against a very aggressive Graves/Teemo and Singed/Darius (that double pull.. >.<). Otherwise, just bully the opposing laner relentlessly, and punish them when they come in and try to CS. You do need to manage heat though, but other than when a chaotic teamfight breaks out, that isn't overly hard to do. The short CD on your skills, and cycling them smartly takes care of that.

Also, I don't find Ori that hard to play - I am usually able to win lane with her, but you do need to put up with a very passive early game, and manage your mana carefully. Leaning ball positioning, and how to effectively zone, and to land her skills takes time, but can be rewarding when done well. She doesn't offer the same amount of burst as conventional AP carries, but she offers a lot of utility, and with the right team comp can absolutely wreck the enemy team. You do need to farm lots though to stay effective.. and I mean lots. That said, she's great fun to play, and is on free week now, so definitely give her a try

A lot of people seem to think Anivia... and I strongly disagree. By the looks of this thread, nobody's a Platinum ranked player or anything, so it's not like your playing expert middle lanes that dodge every skill shot or out play you by a mile... If any of you have seen pros play Anivia (Bigfat's my fave), you'll notice that they sit really far back pre 6. One problem may be that your trying to land double hit stuns at level 3, or harass the enemy out of lane. Anivia is a late game champ, and as infuriating as it is, there's not too too much to do even once you reach 6. Also, Rumble? Really? He's really not a hard champ to play... and if you spec right, you can out damage and sustain anyone with a Doran's Shield, boots or even amp tome! If your just constantly spamming your skills, your doing something wrong. All you have to do is spam Flamespitter as much as you can, take a break every once in a while to let the heat drop to 50, then start spamming again! If your going to go in, get 50 heat, W-Q-E and start smacking their face with all your bonus magic damage!

I don't see how olaf is very hard, to play he has 3 main abilities that have quite a bit of damage output and alot of health.

I have a hard time playing with Rumble and Corki