Fizz or Cassiopiea

Fizz if you're low elo because he is better to carry idiots. Cass if you're high elo because chances are your team won't be feeding morons that go 0-5 in lane and throw the game before you can scale into a monster. Playing Cass in Silver is an absolute nightmare.

well it comes down to a few things

do you prefer fish or snakes? do you like buying boots? do you want mobility/burst or dps ?

cass cant buy boots at all. she has a passive that gives her MS per lvl. fizz is mobile and burst cass is dps mage has to scale cass has range no mobility fizz is melee outside of ult

i feel cass to be good on her takes some skill ( or use to ) fizz is get in get a kill or two and face roll

Cassiopeia is a better champion overall but a good fizz will make you pull your hair out

As mentioned Cass-- high elo (around Diamond+) Fizz - low elo. In overall, asassins work better in low elo, as you can easily snowball on lower skill players. And high skill cap champions being team reliant are satisfying to play in high elo.