Now that I have some time, I will share my new Rengar build.
After playing Rengar for quite a few games, I came to the same realization that Renzor did, in that leveling his W or E first is much more advantageous than Q. After realizing this and knowing that his W scales with AP, I thought about a new build strat for him.
I now have a very unorthodox Rengar build, with early AP switching gears into tankiness (health) and heavy AD for late game. I level W after lvl 1 Q and lvl 2 E. Because I go W first, AP is MUCH more beneficial to him for both ganking power and jungle clears. Many people underestimate his double-shout burst, it also gives him the defensive stats and heal for sustainability, and people get scared ****less when he's still at full HP and they've suddenly dropped to 50%. Once your E starts to level up you can also lock a single target down, double-shout, and then proceed to finish them off with Q.
The key to the build is building items that will give you maximum benefit for the stages of your skills. I still start with a Doran's Blade for your first clear and some HP sustain (and for that lvl 2 counter-jungle gank. ooh so tasty), but my next item is a Kage's Pick. At this point most people say "WTF are you building Rengar?", but ignore them. Kage's Pick gives you that extra early AP boost for W, and gp5 to top it off. This gives you the extra gold you need to pick up a Blasting Wand. W starts doing MASSIVE damage at this point (it is usually maxed at lvl 8 by this stage), and jungle clears are very quick.
At this point you have a decent amount of AP, and the enemy should realize you're going AP (generally considered stronger than AD at this point, but less "ganky"). Here's where we really throw a curveball at the opposing team; we start building health. Grab a Giant's Belt into a Ryjal's. Yes, a Ryjal's. This finishes your early-mid AP building, and now your W is not only doing massive damage, but it's also healing you for way more (15% of TOTAL HP). Your Bola is starting to get leveled, which lets you stick to a lane gank and simply DPS him down (or just lock them up for your partner to kill them), especially with the extra slow from Ryjal's.
Another Giant's Belt into Frozen Mallet will make you extremely beefy, and W will heal for even more. Now I know what you're thinking; "Ryjal's AND a Mallet?!? NOOB!" And usually this is true. But in this case, the huge health bonus from both items makes your healing OP, and because you've turned from the expected "assassin" into a bruiser, both slows are EXTREMELY helpful for allowing you to lay down continuous DPS on your helpless victim. I assure you they will not be getting away with Bola and both slows.
After this, I sell my Pick and concentrate on getting AD (usually BT's). With this build, Rengar becomes a very surprisingly beefy bruiser whose main goal is still targeting carries and DPSing them down, forcing them to die or retreat, but he can actually initiate with his ultimate and not die in two seconds. Because everyone tends to expect Rengar to ulti in and fall like a sack of wheat afterwards, you distract them long enough for your team to rush in and mop up. You might still die, but I assure you you will soak up enough damage for the enemy team to get countered hard.
Again, this is a very unorthodox build, but that is why I find it works so well. Most people will still think "lol troll" when you build a Ryjal's and Mallet, and then drastically underestimate the continuous DPS you can dish out while being a beefcake, and with your new-found CC powers you become a great lockdown/support in team fights. I have had much more success with this build than pure AD or AP. You may not roll faces and snowball with Necklace, but it is a much more reliable strategy I find.
Let me know what you think, and actually TRY IT before you call "troll!"