The fact that nobody can agree on which Champions are outclassed or just under-rated makes me think that every Champion has some kind of use at the moment.
That said, I think that Heimerdinger or Master Yi (When built AD) are the weakest Champions in the game right now. The Revered Inventor is really good at pushing and holding lanes though, so that can be extremely effective in some situations.
I think it's Sion. He was the first champ I purchased and I still like him as a champion, but his kit is just plain awful. And AD champ with one AD centered ability (ult), and an AP champ with two useless abilities (E and ult). Needs a rework pronto.
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Ok, I play most of the underplayed champs, so there....
Sejuani on lane (top) have a horrible early game, so she's terrible there. In the jungle her clear times aren't bad, however her ganks pre-6 aren't really good (she's got too little damage). In other words she's quite useless in early game. She's quite useful in late game teamfights though (disabling, slowing every enemy in the team fight + DMG from her W while enemies are on permafrost isn't bad).
Karma is a terrible bot support. However, she's pretty decent on mid (AP build), since she wins quite a lot of match-ups, and then is very useful at keeping carry alive in teamfights, while doing nice damage to enemies. She also requires more skill than it seems like it, both from her and her team.
Trundle on lane (top) isn't bad. His sustain is good, and he can't be ganked easily (I know it doesn't seem like it, but it's true). Also, he makes ganks easier for jungler. In jungle I find him pretty much impossible to counter-jungle... But he needs level 4 to gank effectively, and even then his ganks are not the best (enemy team must REALLY out of position for him to work). Also, he got quite strong early, but only mediocre late.
Heimerdinger is the worst of these champs honestly. I mean, sure. He can counter-junglea bit (not being jungler himself), can do Dragon fast (preferably with the help of the jungler at level 5), and it's very hard to push him out of lane, thanks to his insane pushing abilities. However, he is bad at any fights with other champions. He's squishy, he doesn't scale all that well, his stun is the goddamn hardest to hit skillshot I've seen (a bit easier on ulti) and his turrets are easily destroyed (and they are the source of most of his damage). Still, if you've got team that know what he does, you can take advantage of his other uses (split-pushing, counter-jungling, and taking all the dragons).
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