This is depending entirely on when is an assassin at their most powerful.
Kat and Yasuo have terrible, terrible, TERRIBLE laning phases (made a bit better by the fact that the mana-mages got kicked in the groin). And are at their strongest in teamfights, they are often hoping to get fed off a dragon fight or such.
Akali is at her strongest the moment she hits 6, before this she is rather weak, and easy to bully around, depending on the matchup and the situation, by the time she hits 6, it may be too late. Since she isn't particularly strong in teamfights she's a bit of a burden once she's fallen behind. She is similar regarding this to Talon, but Talons AoE capabilties still allow him to contribute something to a fight even when behind.
Le Blanc is different from most assassins in that she is at her strongest from the get-go, her lvl 2 cheese is very strong, she is a strong laner, and a strong roamer. She is very likely to snowball if you let her, but very weak if you don't. She was similar to Fizz in this regard, but Fizz has been severely nerfed since and doesn't really stand out at any point in the game.
Ahri is very strong at all points of the game, her lane phase is pretty oppresssive since she has decent poke, and a brutal all-in if you take a charm to the face. Her teamfighting is very strong by virtue of her CC, poke, mobility and burst, she does it all, but she is rendered useless by her charm being dodged.
Zed is a funny one, he doesn't NEED to get fed to be relevant since his kit grants him free stats, and his laning phase is very safe, but unless you are eating shurikens to the face all day long, he isn't really a strong laner who can snowball off you.
Diana is more of a bursty bruiser and Yi is a carry-type jungler, they aren't really assassins since they can't do the "Go in, kill someone, go out" shtick.