My build isn't the very best at the moment, but this is while I still experiment with Rengar and go on to make a better build! Improvements and thoughts on how to make it better are always welcome.
In the masteries section, I allocated those points because it would give Rengar more damage and speed that wasn't given to him in the runes.
I chose these items to complement Rengar's natural leveling. In the regular build he is built to be off-tank and still do some serious damage. In the jungling build, it is made so that Ren has some more armor and life steal to do some nice jungling.
The skill sequence is the same in both because I think both the Q and the W are the most important skills in Rengar's skill set. Giving him some good damage off of the Q and some good armor/magic res off the W.
These spells give rengar some good diversity. With flash, he is given a quick escape when necessary and with ignite, he can finish the job quicker.
This build was made off my little experience with Rengar. I will constantly update this build and strive toward the best build I can make.