So, first I will discuss Corki. According to my boyfriend Corki is an unviable ADC. I tried him out and ripped through everything. I don't see any problem with him. He has a good amount of damage, is good with mana, and has a perfect escape. What's the problem with him? I see him as a viable ADC who can do a lot of damage, maybe even out damage some ADCs.
The second I will talk about is Skarner. Skarner is a very strong jungler and has good potential. He just isn't played. His Q gives him strong damage, and his W gives a good shield with movement speed for escapes. Build an Iceborn and tank. He may have low gank potential or a slow jung but he is viable, right?
Third, Yorick. Yorick, I like but I don't see him as viable as he could be. He has mana probs because he is a huge lane bully but his mid/late game crashes. I wish i could say he was viable but he isn't unfortunately.
Fourth, Poppy. Poppy used to be viable but she is just downhill now. She has terrible early game farming, a slow mid game but a destructive late game. I enjoy her but she can't hold her own all game.
Fifth, Galio. He is a tank and is very strong for AP. He has a shield that gives him health and is hard to stop? His fault? Well, according to my bf it is his ultimate "his only useful move". But, in my case I find him completely strong. His Q does good damage, his W is a good source and his E is amazing, and his ult provides good CC for your team.
Sixth, Graves. I find Graves to be really strong. He has a lot of damage and an okay escape. How isn't he viable? I'm not sure on him.
Rumble, our little hero. He is getting reworked to be better, that's great <3 edit: Rumble is just unused for whatever reason and new skin soon, bringing out some rumble players...
There is Urgot. Urgot has hardly any range or damage, he isn't viable unfortunately. I hope to see change on him soon.
Alistar. He is a huge tank and can provide good dmg and heals if built right, but what is his downfall? He crashes hard sometimes. I wish I could say he was viable but he needs worked on...
Sion. He is getting worked on thank god. He is weak and unviable at this point, S4 has introduced new items and new meta, making Sion not so useful, which explains his lack of playing.
-This isn't saying Karma is unviable but why doesn't anyone play her, she is amazing and does a large amount of damage.
Edit: Why isn't Zilean used...?
Viktor is unused too but I'm not sure why s:
Don't see many nunus...
Is Talon unviable this season?
EDIT 3-24: Okay, i found out Talon. He isn't very viable due to the fact that he is an assassin and he has to go in solo while the rest of the team tries to work together.
Also, these are my experiences and opinions, i am delighted to be informed of the things I don't know, but please don't be rude about it.
If i think of more I'll add on, but what are everyone's thoughts?