I pretty much gave up on Top Rengar because of some major changes, maybe someone else picks it up or has picked it up.

Teleport

is a nice spell to have pretty much the entire game. You can TP-Gank Bot with your Ult, you can help contest Dragon, go splitpush a lane, TP to the minion attacked by tower while sieging with teammates. Its an all around nice spell to have.

Flash

is a very, very, very good spell right now. You can escape over walls, Engage-Flash onto an enemy carry, do some fancy combos. It's a really good spell, you will very rarely want to pick something else.

Ghost

is another option, although I don't recommend it for usage instead of Flash. Rengar doesn't have the mobility to spare Flash, but a Ghost/ Flash could work out quite nicely for chasing the Singed.

Exhaust

is recommended against high-burst champions like Pantheon. You want to use it when they do their combo, since burst-champs often have high Cooldowns early on, which means that they did far less damage to you and stand there without something left. Easy kill for you.

So, you maybe noticed it in my mastery tree, but I did a 0/18/18 mastery tree. Which is obviously not possible. The point was that Stormraider's Surge and Grasp of the Undying are both very useful. Stormraiders will give you much more mobility when you have your items, but in lane Grasp will help you out with it's lifesteal. I see them both as evenly good, so decide yourself.

Stormraider's Surge
Grasp of the Undying