Electrocute is a fantastic assassination rune on Rengar thanks to his ability to almost instantly proc it with a Leap->[Q]->[W]. The extra burst allows you to secure your kills in a single rotation of spell.
Sudden Impact is a no-brainer on Rengar. He is a natural abuser of dashes thanks to his passive. With good positioning around bushes you should be able to keep the buff active at all times granting you permanent free Lethality.
Zombie Ward is a safe a reliable rune on every Jungle thanks to the nature of their vision control and ability to deep ward.
Ultimate Hunter is a great replacement for Ultimate Hat on Rengar, allowing you snowball successful [R] gank leads into more kills in the mid game.
Celerity's Movement Speed and bonus AD on Rengar is great for empowering your ability to content early scuttle crabs and scrap for fights in the early game.
Waterwalking further empowers your ability to contest scuttle crabs and scrap for fights in the early game as well as synergizes well with the extra movement speed from your [R].