Gain 10% CDR when you reach level 10. Excess CDR becomes AP or AD, Adaptive.
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.
Relentless Hunter is a massive buff to Rengar's [R], allow you to travel much further distances with just a few stacks. The bonus Movement Speed also allows you to get around the map to meet fights in a timely manner.
WhyThe Ultimate Hat
Rengar becomes a scary threat when he is ahead and his [R] is up. More chances to cast [R] means more kills and that's what Rengar is all about.
Transcendence allows you to build Black Cleaver and Lethality items freely without fear of wasting gold on over-capping CDR.