Nautilus is a stronger pick atm due to tank strength in the current meta. He's typically a Jungler but with his ability to lock down any target he chooses he is being played as a Support the same way Leona is. For that, he's a good option. His skill ceiling I wouldn't say is low, but it's not because his kit is hard, it's that you have to select the best target to lock down instead of hooking and ulting whomever is closest willy-nilly.
Elise is pretty much Jungle only and not a strong pick at the moment. Riot has their eyes on % health mechanics (which is her primary damage source) so she could get beneficial changes at any time to bring her back into the meta. High skill cap due to being a transform champ, but needs a team to back her up (like Orianna ball or someone to chain stun with like Morg).
Rengar can go top or jungle, but like Elise, is not a popular pick atm. I just bought him so can't really say how strong he remains in this patch, but he looks fun. It's obvious that it takes skill to work him effectively, so if you just want a high-skill ceiling champ to master, get him.
TL;DR - Nautilus for being good in this meta, Rengar for higher skill requirement