It all depends on what you want to play!
Fizz: bursty AP assassin, mostly played mid and jungle, nice mobility
Vladimir: AP caster that excells at sustained damage, healing himself and giving advantage to his team in teamfight
Shyvana: A good jungle that can be built either as a tanky bruiser or and on-hit effect fighter
Cassiopeia: A hard to play mage that can bring good sustained damage to her team
Kassadin: Just like fizz expect he isn't played jungle and is more powerful late game, but has a bad early game
Leona: Tank support who excells at locking down a single target with her stuns and good engager
Anivia: AOE late game mage who deal great damage and has good utility, but she is hard to play due to her wall needing good timing and her lack of mobility
Brand: A mid/support burst mage that deals a lot of damage with his passive and synergize well with spell effects such as Rylai's slow or Liandry's DOT
Corki: A kind of AD caster that deals great damage but is reliant on landing his ults and needs good positioning
Sona: A great poke/sustain support with a nice ultimate, while she doesn't excell at anything, she isn't bad at anything either
Hecarim: Good bruiser who brings nice damage and initiation
Irelia: single target focused fighter who deals tons of damage and has built in sustain, CC reduction but no AOE damage but her ult
Kennen: can be played as a Mage or a marksman (more often mage), has a good teamfight ult and cool passive
Sejuani: All in tank that deals some damage and has great CC, amazing initiation ult
Swain: A mage that can lock down multiple foes but who deals more damage to single targets due to 2 of his spells being targeted and one of them increasing damage you deal to one target
volibear: great jungler, nice CC and damage overall
Gragas: tanky caster, great CC but needs to land his ult perfectly in order to push foes in the good direction
Graves: Unique Marksman that deal a ton of AOE damage but requires good positioning
Wukong: Bruiser that deals great damage and has good initiation/follow up with his ultimate
Quinn: Deals ton of damage and good Map presence when she hits 6, but most of her damage is single target
Lissandra: Tanky mage that can initiante teamfight (probably the only mage to be able to do so)
Draven: heavy single target damage marksman that requires a lot of skills and positioning in order to deal damage
Illaoi: Tanky immobile damage dealer with very few CC, but lots of sustain and AOE damage, she requires a team that can engage for her so she can follow up
Gnar: unique marksman/tank playstyle that requires good timing, he can initiate team fights and deal AOE damage/CC while in Mega form, while mini Gnar focus on kiting and single target damage.
Hope this helped! If you have any question just ask :P