Surprised at all the Draven answers, I used him twice on his last free week. Easy mode.
I'd say Ashe & Vayne, with Kogmaw nearby.
"Who is/are the hardest adc(s) to play effectively?"
There's a lot of micro management, choices and risks involved with playing Draven well. Catching the axes is the easy part but knowing when you should and shouldn't takes a bit more practice plus he has a global ult which are never exactly the easiest type of ults to master. Also his steroid is dependant on good axe management which adds another layer of risk/reward to his gameplay.
When I time all my axes perfectly w/ constant W refreshes and clutch on the tip Es, it's like I'm God. When I don't, I feel like **** worthless.
If we're talking a strict "what carry is the most difficult to fully optimize their potential" then Draven is the answer.
That. Vayne is still difficult because of the necessary repositioning to land a stun at a clutch moment or instagib the enemy carry in a tumble. Draven; however, needs to catch those bloody axes with spells flies all over the place.
Vayne can fail a tumble nuke and recover with that blip of stealth. Draven screws his combo and he is a free kill.
Fiora is mai waifu. :O "I read this as a Bannanable Offense. I was like, what ksing with raka?" -Kirby1207
"hard" is a subjective term because not only are their general playstyles almost exactly the same, but at the end of the day it's just last hitting and right clicking people while making sure you aren't getting jumped on by the whole enemy team. you can make every role sound simple like that, because this game is really easy to learn and be decent in, there are just things in the game that are less easy.
I don't think Vayne is that hard because she doesn't require doing anything that other ADCs dont also require. Yes, Graves/Corki/Ezreal have really stupid "get out of jail free cards" that Vayne doesn't have (sorta, if you view tumble as a decent escape, which it can be I guess), but positioning is still important for them, and once you get half your build with Vayne, fuggedaboutit. Her only difficulty is in lane phase and if you can just manage to not feed and stay within 30 cs of the enemy carry your chances of winning are extremely high.
If I had to say any ADC is the least easy to get the hang of, it's Caitlyn, if for no other reason than she is easily the worst ADC in the game and if you can consistently carry a game with her, you've "mastered" the fundamentals of ADC and would probably be doing better, or at the very least just as well, if you played any other carry instead.
Honestly, the more I play the game, the more I become convinced that there is no such thing as an "easy" dota. I will not argue that some dotas are easier than others, but when you compare dotas to virtually any other genre except maybe RTS, you will find that even League has an absurdly high learning curve, a reletivly high skill cap, and is extremely unforgiving (once again, we are talking about general standards, not dota standards). In short, while League may be easy compared to other dotas like HoN or the original dota, to call it an easy game would be misleading at best and often an outright lie. Also, Caitlyn is not a bad ADC for one reason and one reason only, her auto attack range. All ADCs will be atuo attacking quite a bit, and how far away you can be from the enemy team and still auto makes a huge diffrence. I am well aware that Trist can do this too, but Cait can do it from the get go and outclassed does not mean bad. Cait has a few other things going for her, her net makes a decent escape, traps are basically free wards, q is an excellent harass tool, and her ult is decent for after teamfight cleanups. But at the end of the day she's mostly going to be auto attacking, which she has an advantage in because of her range.
How does Riot keep it's balance? That I can tell you in one word! http://www.youtube.com/watch?v=gRdfX7ut8gw