The last nerf on Rengar with his Bola was actually pretty significant to his general play-style. Although everyone usually plays him as a straight assassin and builds as a burst champion, Rengar can be built and play extremely well as a bruiser such as Renekton. Although you may not just instantly burst anyone, you can still do a ton of damage while being able to take it at the same time, and further help your team in teamfights. Hydra doesn't even need to be a key-core first item the most feel the need to rush. Although even then, I would always include Hydra at some point regardless in his build as a first, second, or even third item if need be just because it really does work well with his kit. If you would like to try it out, here is my general build for him as a tanky-brawler.
Hydra, boots (whatever is needed for the enemy team comp), Maw, Dead Mans, Black Cleaver. And the final item can be what you want. If its a heavy ad team, build sunfire or thornmail, if they're heavy ap, build spirit visage. if you're just really far ahead, build an ad item like deaths dance. Obviously you're not going to play Rengar into just anyone, so don't think that. He can be good against alot of champs, as long as you play really cautiously instead of aggressive. Most other top lane champs have the edge on him at level 2-3, so poking with jump and q at level one is always good.
~ Yes I play him top lane.