Master Yi, hard to master but wrth it..

Anivia

My name is cassie

deffo Lee Sin

Already mastered kog'maw, and he can easily be countered, not saying he isn't godlike in late game, but his movement -and burst isn't as high as a late game good vayne imo. Vayne's abilities are more fit to her survival than kog'maw's.

But as you said, these might just be my fantasies, but that's my opinion anyways^^

Orianna
The hardest and the best^^

According to Dan Dinh, it's Xerath. That's just his opinion though. Quote from his AMA thread ( http://na.leagueoflegends.com/board/showthread.php?t=1422997 ):

"Personally, I would just spam 30 xerath games right now to get to platinum from a fresh account. He has a huge learning curve and I think I've gotten over it."

I believe him, although I have played only one match with Xerath myself. I didn't do well, but this alone proves nothing ofc.

70% slow and like 1000 range attacks taking 30% of tank' health every second with 2.5 ats.
How can you counter that? Vayne needs to be in their face to do anythingt.

Not saying vayne isnt good but its not that good in solo que at least.
Kog, cait and graves can deny half of his farm as well unless he is lucky to have soraka.

True true, but his slow can be avoided quite easy, once you get close to him he doesn't have any escape abilities. Although I must say that vayne's laningphase isn't the best, as you said. But with a soraka and some runes you'll be able to get the farm you need.

And now to some offtopic. I like these arguements that comes from a simple question "Best champion to maste but hard to play". Gives me a different PoV of some champs and I'll improve my skills in-game, so thanks Lieto for sharing your thoughts with me ^^

^ Simple fact however is that vayne will loose most of her health she gets into attack range with kog.
Also he can throw his slow all across the lane to completely shut it for a second so there is no way it can be avoided. That may look like nothing major but those little things make champion better or worse.