I run Rengar 21/9/0 with flat AD Quints, flat AD marks, flat armor seals, and either scaling MR or cooldown reduction Glyphs.
I start hunter's machete and 5 pots. With 2 other people and great assistance, you can start at Wolves and clear them before the blue buff even spawns. After grabbing blue buff, I go Wraith Camp, Golem Camp, and then Elder Lizard (This way, your smite will have cooldowned by the time you start fighting Lizard). If I'm feeling confident, I'll clear either Wolves/Wraiths one last time and then b.
I build the Madred's and return to another camp until I have 3/4 Ferocity stacks, where I'll then immediately look for an available gank. Rengar realies pretty strongly on the early kills, and you won't be even half as effective without them.
If you're having trouble ganking and the enemy team doesn't have much cc, grab Mobility boots right after you finish your lantern to help you sprint into lane faster. With Mobility boots and ghost, there is almost absolutely nothing they can do to stop you from charging into the lane, slapping them with a bola, roaring in their face, and either bursting them down with an Enhanced Q or rooting them to the ground for you and your ally(ies) to pick them apart.
If you're fed and are getting many assist/kills due to successful ganks and plays, pick up a bonetooth immediately; otherwise, rush a Cleaver for damage and some sustain.
My usual build usually looks like: Wriggle's, Mobility boots (with either Furor or Homeguard), Bonetooth, Black Cleaver, Infinity Edge, Frozen Mallet.
If the game drags out long enough, sell the Lantern, buy a SoTD, and proceed to dive and destroy the enemy carry with your 100% chance to crit for 250% damage.
Other situational items you might want: Maw of Malmortius (Always rush the hexdrinker if the enemy AP is fed and nuking everything. No exceptions.), Guardian Angel, Bloodthirster.