Rengar is a very strong top because of his ability to escape ganks at 6, and being able to deal burst damage while also having sustain. This build revolves around hiding in the bush, spamming your skills to get 3 fury charges and jumping on the enemy to quickly Q->W->W->E and run back into the bush.

The summoner spells I've chosen are for assisting other lanes, and chasing/escaping. With Rengar's ultimate and ghost, it is nearly impossible for anyone to escape, and it also provides the ability to quickly escape ganks as long as there is a ward nearby. Late game, using your ultimate and ghost makes cleaning up team fights very easy, and allows you to quickly jump on important targets in the back line mid-fight .

The summoner teleport should not be used to go back into lane, but rather to gank the bot lane when the enemy is over extended. The support generally wards for bush control, so during the laning phase it is easy to position yourself behind the enemy, cutting them off when extended too far.

The items I've listed are generally for AD tops, against AP tops (ie Malphite), I generally prefer to get an Abysal Scepter early, as it mitigates a large portion of the damage you take, while increasing your W damage.