Not Updated For Current Season
I see panth is almost never used these days and hope that maybe this build will get people to start playing again.
This is also dependent on you being good enough to get a couple kills in the beginning. A few kills will go a LONG way and you'll be indestructible (well you'll kill them before they kill you) by mid game.
This is my build that I always use in normal 5x5 and 3x3 queues. On average, I have many games where I go 15-1 and the games end early. However, it's all a matter of whether or not you know when to nuke the person. I'm not much of a writer so this is all I'm going to say. Your best opportunity to gank is at 6 and always be on the lookout for any enemies who extend themselves past your creeps. Should they overextend you can teleport in and smash them for an easy kill. Also, if once you are strong enough to end them with a quick combo, you can flash -> W to cover a hell of a lot of ground.
Your main fighting will be last hitting minions and Q harrassing the enemy and when going for the kill, Q W E or W Q E and then hit them at the end of heartseeker and if they arent dead by then your Q should be off cd and Q them again
As for the 6th item you can choose between some armor pen (last whisper) if needed or another bloodthirster.
I've got to give all of credit to DEWO as I took his skills and masteries and item build essentially and just tweaked things (teleport, warmogs, last item).