Riven is, hands-down, one of the best champions in the game. Easy to learn, but hard to master so she doesnt bore you, EXTREMELY strong in the right hands, no caring about mana or such sh!t, really fun to play, nice design, good background story.
I can only recommend her.
Fizz I didnt like at first, but the more I play him the more I like him, and he fills a pretty unique role as a meelee mage (only others are Kassa and Rumble, which are both completely different in playstyle), and his scaling is so incredibly sick he can absolutely own anything in lategame. Very rewarding to play as well, but not as easy to pick up as Riven so I only recommend him if you dont mind putting time into learning a champ.
Lastly, Riven is out a lot longer so people got used to her already, whereas Fizz may still receive balance changes, and most havent adapted yet, which makes Riven the "safer" buy.
All in all, though, BOTH is really the best answer to your question