Rengar has great potential for top lane. He has pretty good harass with his passive and savagery. Good sustain with full ferocity battle roar. I decent chase and escape with his bola strike. Pretty good escape with Thrill of the Hunt assuming they don't have a pink ward or oracles.

He is new and hated on the forums for some reason. I quite enjoy him top lane, he can bait really had under his tower with his battle roar

I use flat attack damage runes since he has two abilities that scale off of attack damage. He currently has the highest ratio attack damage scaling on his full ferocity savagery. For each point of attack damage he gets 2.5 damage.

Flat armor runes for the early mitigation.

Magic resistance per level for mid to late game team fights. They are better than flat magic resistance runes at level 9.

Attack damage quints for the same reasons as marks.

Almost standard 21-9-0 masteries, I replace Lethality for Demolitionist since he doesn't benefit much from crit.

I start just about every champion with boots and health potions. Depending on how you do in lane, I'll either get his bonetooth necklace right away or get a phage for the extra durability. I've started building Mobility boots on Rengar, he can roam from top and gank other lanes with ease.

After this, gauge where your lane opponent is and what kind of damage they're doing. If you're up against an ad champ, build guardian angel sooner, if ap, build hexdrinker fast.

You should be around late game now. Build a guardian angel, the passive on it is amazing and can easily turn team fights around and the armor and magic resistance help a lot. Then finish trinity force, if you're fed early feel free to finish this sooner.

NOTE:

Building attack speed on Rengar isn't needed as his Savagery can provide up to 150% bonus attack speed and he already has a high base attack speed.

Maxing his Savagery is pretty self explanatory. More damage, more attack speed.

After this, play it by ear. Do you need extra durability? Max Battle Roar. Do you need more chase potential? Max Bola Strike.

Of course always max your ult as soon as possible.

When playing top lane, make sure you ward and watch your map.

If you're getting bullied in lane and losing, try sitting in the bush and jumping to last hit. It seems to have worked pretty well for me, most people won't have the reaction time to harass you before you're almost back in the bush.

If you're winning the lane get to a 4 stack of ferocity and then just last hit with auto attacks. That way, when you go to harass the enemy top you can double savagery for some good burst, bola strike then walk away.

In team fights I usually wait for an initiate then use my ult to jump on their AD carry and either cripple them so they have to retreat or kill them. After that I usually jump on the next priority target, etc. If you're being focused try and get a full ferocity battle roar off to survive a little longer, it will also grant you extra armor and magic resistance to improve your damage reduction.