Official tier list of easiest and hardest classes in the game

I'll start with the most skillful ones

Assassins: Assassins take a lot of skill, since their mobility is tied to their kill ability, and they are really squishy. While their ability to deal damage can be higher than other classes, the skill that it takes is beyond what is asked to an average player. Examples include Rengar, Zed and Talon.

Bruisers: Bruisers are slow and, though they deal decent damage, they are usually kited to oblivion. Most bruisers are hard to use since they are easily kited and are not as tanky as true tanks, also lacking, usually, lots of CC to compensate for that. I believe they take a lot of skill in general. Examples include Irelia, Garen (easy to play but highly kitable and without ult his damage is really lackluster, also too little cc) and Darius

ADCs/Marksman: With the exception of Urgot, which should actually be a bruiser, ADCs require a balance of planning and mechanics. While ADCs don't take that much skill in general, the fact that they are not protected correctly and that they lose damage or utility if they desire selfpeel, in SoloQ, makes them relatively balanced. A good ADC can win games, and a bad ADC can ruin them. It is one of the most fair classes IMO. Examples are Graves, Sivir and Ashe

Tanks: Tanks can absorb a lot of damage, have high base damage and usually can disrupt and win teamfights by their presence alone. Tanks don't take skill to be used and have usually easy to hit CC. While not the most unskilled class, on SoloQ it helps restore order to your team and prevents the enemy from being useful for a while, which is extremely helpful. Examples include Cho'Gath, Nautilus and Sion. Note that some tanks are supports, which lowers their skillcap/floor even more, but not at the same level that true supports.

Fifth place is a tie between mages and supports. Both are really unskilled for various reasons.

5a. Mages: Mages take little to no skill to be used effectively, have almost always hard CC and some mobility in their kits. Their damage output, be it constant or in a burst, is higher than all other classes, and can easily destroy a target even if really behind. Itemization is beyond godlike, having a 2.5 second invulnerability, tanky items that provide above average damage, poke and shields. Examples of this cancerous class are Diana, Veigar and Lux.

5b. Supports: Supports effectively are useless under almost all circumstances. In the few times they can be useful, they take no skill to be used correctly and are just a waste of a slot in a team. They're picked by noob players whose only role is to ward the map and provide auras for team. Even a beginner can play effectively support. Examples include Janna, Zilean and Taric.