If you like a challenge, Azir has a really high skill cap and is very strong once you get good at him. Lissandra is fun if you like playing mage, but want to be able to initiate fights. Rek'Sai is a very fun champion thematically, you get to feel like a submarine. Ekko is very fun too, he really rewards you for smart and flashy plays. If you want to be able to carry games I believe that Azir and Ekko have the best potential for that, Azir just because of how strong he is, and Ekko because you can build ap or tank, depending on what you need, and he has the mobility / cc to make some very powerful plays.