Honestly there are some heroes that if I get i'm 100% confident I can win a game, then there are characters that I "think" I'm good with and get some kills but have no impact on a game, and then realise that I'm actually terrible at them
The top contender is Riki -Proably surprise some people, I love playing him don't get me wrong, but I'm not aggressive enough or offensive enough with him, totally switch off. Almost alwyas lose if I play Riki, still have definativley worked out why.
Wisp - Is just frustrating to play, situational and I can't play him to save my life
Meepo I use to be able to play in original Dota, but now I can't play him at all. To a lesser extent Chen also, although I am getting much better with him when playing him as super offensive (not as many yells lol)
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Give Lich a try as support (if you don't have one) and he becomes amazing. I mean there are always times Lich is going to suck, he's got one of those ultis (similar to bloodseekers) that's easily avoided or negated. With the right group, he's devastating. With the wrong group, he's bad after early game (even before). You should play him support, can be lots of fun