I've been playing poppy for a while, and basically, you can kill anyone with devastating blow. I've tried many builds, but eventually I just made my own by meshing them together. Have fun. The only thing I have against poppy is her standard skin... she's pretty ugly. It's also very depressing when you type /joke, and she replies "Jokes? I don't have any jokes" :( I like jokes.

The runes are pretty straight forward: the armor and magic resist helps with survivability early game and the armor penetration lets you hit hard.

Poppy is pretty weak early game, so instead of going for mana regen in the support tree, I grab the extra defense.

Pros
You win
hits hard
ult is fun to piss people off
can 2v1

Cons
squishy(except passive helps)
horrible early game

Some people might disagree with the order of my items, but I find this way to work the best. I get a vampric scepter early because it's VERY useful for regenerating your health as you push so you can be ready for your next kill.

Poppy has a horrible early game. Don't be too aggressive and just chill back and farm. Make sure you're lasting hitting creeps, and if you don't have a jungler,(why don't you have a jungler?) kill the neutrals in your spare time.

Basically i'm making this guide for someone who knows how to play and just wants a good build instead of reading an extremely long explanation on things you already know how to do or what works best for you. If there's anything you would like me to add, just tell me and i'll try to get around to it.