Here are my tried and trusted builds for effectively using Poppy. I mainly use the offensive build but in certain situations the defensive build is 100% necessary.As with the rest of my builds this build is very subject to change based on preference; especially the runes, masteries, and summoner spells. Please don't down vote just because you want to change these things, this guide is just guidlines for an effective build that I wanted to share. (sorry for any spelling or grammatical errors)

Well for both builds I took flat health in quintessences which are great for dominating the laning phase. I took critical chance for the rest of my runes in the offensive build because there is nothing like critical striking with Poppy. In the defensive I took Critical chance ability power and more health to make a more rounded rune build.

I took very offensive masteries in my offensive build because well that is the most effective page for damage dealing with Poppy. My defensive masteries are a more standard 9-21-0 which improves my summoner spells and makes me tankier.

In items trinity force and infinity edge are a must on Poppy. The rabadons from the offensive build is underestimated because both of Poppy's main skills improve off ability power plus it adds damage to her from sheen which makes it a quintessential offensive item fore her. The bloodthirster is nice for staying alive in team fights and owning 1v1. The guardian angel is a very nice last item, not because you will be dying a lot, but so you can stay in team fights indefinitely.

As you can see my skill sequence prioritizes Poppy's damage spells and obviously takes her ultimate whenever possible. Her W is useful so I take a few in it along the way.

IN my mind exhaust and ignite are a must for her. Your W practically gives you ghost, which is great for chasing and quick getaways. Exhaust helps while chasing and also helps you in 1v1 where it scales nicely with your ultimate. Ignite is extra damage and can take down enemies who would have otherwise gotten away.

Pros:
high damage
burst damage
consistent damage
fast
tanky
tower diver

Cons:
Often targeted
No AOE
Causes early enemy surrenders

Poppy is a much more individual champ but she can still excel in taking down key enemies quickly in team fights.

Poppy can last hit very well with her low cooldown and low mana Q. The problem is that she has no AOE so she cannot quickly take down hole minion waves until late game.