Looking for a good champion to master

So I've been playing Akali a lot and I've managed to get her to level 5 mastery, however she doesn't have that high of a skill cap so I've gotten quite bored. I want to try to main a champion that has similar gank / kill capabilities as Akali but has a higher skill cap. I've been recommended Riven, Vayne, Yasuo and Rengar. Are there any other champs that are really good when mastered? Which one of the ones I've mentioned do you guys recommend?
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