Talon, Shaco, Akali, Katarina (maybe).
Talon is too easy to play, mechanically. There's also not that much counter-play, he just jumps on you and bursts a lot, while going invisible for a bit. He's pretty bland, from a design point of view, and he's not fun to play against when he's overtuned and/or fed.
Shaco has a lot of issues. His Q is too safe and frustrating to deal with when he has 40-45% CDR, and his W is frustrating to deal with (invisible traps that fear you and deal a lot of damage if he builds AP). His passive, E, and R are mostly fine, but Q and W could benefit from some changes. Here's an interesting (and experimental) thought: What if his W was his clone, but on a fairly long CD and without any damage (or low damage)? And he'd get a new ultimate, something related to Jack-in-a-box, perhaps? Perhaps he could spawn a lot of Jack-in-a-boxes around an area, or something? I don't know, but there's room for change in his kit.
Akali is poorly designed. She hides in the shadows and then jumps at you over and over again. I'm not really sure what to do with her to make her satisfying both to play and to play against.
I wasn't sure if I'd put Katarina on the list, but I do think she has some issues. She might be a bit too easy to play right now (mechanically) since she has 0 skill shots. Perhaps her E should be a targeted dash, similar to, say, Lee Sin's Q? She shoots a really fast-moving projectile and moves to the first enemy champion hit, and if she doesn't hit a champion she moves to the first minion/monster hit, and if she doesn't hit any of those she moves to the first ally? Or something, I'm not sure. It would add some counter-play (rather that her being able to button-mash to pentakill in some cases) while also making it possible to buff other parts of her kit (more damage on E, perhaps?).
I don't really view Evelynn as an assassin, she's more of a bruiser/AP bruiser, depending on the build. She could benefit from some changes though, right now she's not very mechanically demanding.
I personally find Fizz' E really frustrating to play against, but I doubt Riot would change that. Same with LeBlanc's W and passive and Zed's insane zone-control and mobility. I will admit that it takes some skill to be good with Fizz, LeBlanc, and Zed, but they're still problematic since they gain so much mobility which lets them counter-play you more than you can counter-play them, in many cases.
