I'm going to just let you know, there aren't any particularly good top lane assassins. Except Quinn, though "assassin" is a rough fit for her, and that's the closest word you can use to describe her. You could say Yasuo, but he just got nerfed and isn't really being played like an assassin right now. So if you want to main assassins, you should probably go mid/jungle and not mid/top.
Also important to note is that by the end of this year, Riot is going to release one of their "roster updates" for the assassin class. This means that some of the most worthwhile assassins to learn (
) are going to get farily significant kit changes, and most other assassins are going to get some minor changes.
In mid lane, you are probably going to want to look at
and
. Ahri is one of the best all-round mid laners in the game right now, and is fairly useful even when behind. Zed is the traditional "muh skillcap" "outplayed" champion, and while he isn't super strong right now, Riot loves him, so you can always count on him being eventually buffed again, and you can be fairly confident that he won't recieve massive changes.
In the jungle, look at
and
. Its a versatile set of champions and all of them are fairly strong right now.
A final thing I want to say is that assassins, probably more than any other class, reward you for committing really hard on one specific champion. I have a friend who is a diehard Rengar main, and the difference between his Rengar and his other champions is insane.