Obligatory Warning: As with most/all builds on Mobafire I have to start with the fair warning clause that lets you know that this is merely how Iplay as Poppy. It is in no way my way of saying it's the BEST or ONLY way to play her. Also, I'd like to give credit to HIACliff,DigDug86 I used their builds for a long time with slight variations until I came to this one. Also, Thanks Dellingr00 for the cheesy "Pop-fly" title.

I'll post some of my Win scores on here, as with all Mobafires you generally just post the stuff that makes you look incredible. And, while I do have my bad games as much as anyone else. Even my bad games usually go pretty well for me. Here are a few, I'd like to see how you do with the build good and bad.
Even a defeat isn't all bad

Lastly, this is my first guide and I'd love some criticism. Keep in mind that NO mobafire build is a sure-fire thing every time, changing out items based on the other team's structure is very important.(ex. An all AD/Melee team and you should probably not be buying the Veil).I use this build for BOTH my 3's and 5's matches. While I do play mostly 3's I have more recently started 5's and found it to work pretty well.

My runes are probably one of my weakest points (which admittedly probably isn't the best thing) I believe I copied my runes off of an early "Two-hit KO" Poppy build, and have not changed them much since then. Any advice would be greatly appreciated, but as far as I know having that armor pen is pretty huge for Poppy, she works as a burst damage character that gets in and out of the fights quickly so you want to be doing the most damage when you're IN the fight, penetrating the armor is a good way to go and makes up the majority of my runes. Seal of Evasion is helpful for that extra bit of survivability and works wonders with Poppy's passive. If you have 10hp after reducing an enemies damage by 50% and a minion fires at you and you dodge it...you'll be thanking those runes. Lastly the Glyph of Focus allows for you to deliver more Devastating Blows, more wall slams, and have your Ult available to you a lot quicker (which is also huge)

My masteries are 23/6/1. Attempting, once again, to get the most burst damage with some survivability is the key to how I play poppy. The single point in Haste is important because of the tower diving and general GTFOing you'll do with this build. Poppy is designed to do some crazy stuff with her ult, and being able to get out of danger before it wears off is very important.

First off, I don't rush Sheen, nor Trinity force, though Sheen and Trinity Force are still in the build and still quite important. I just found that with how Poppy plays early game she either gets harassed too much or (due to her Heroic Charge) she over extends or merely puts herself in an uncomfortable place. By grabbing a regrowth pendant and a health pot, after she takes a few ticks of damage she can get her health back without slow dwindling down and being forced to recall earlier than expected. The pro side is getting a very quick and very CHEAP Philosopher's Stone (which is to be flat out sold down the line). The con of course is that without the early Sapphire Gem you'll find yourself running out of mana a bit quicker. Early game is rough for Poppy with all builds, either she has health or she has mana issues, try to figure out how to keep yourself up the best.

You can usually buy the Philosopher's Stone, Boots of Speed, and a Sapphire Gem on your first recall. Next time you come back to battle you're still not much stronger, but you have a lot more survivability. Poppy's abilities by themselves are already rather strong, this is why I don't rush the very powerful Sheen. Her stun and her D-Blow are enough to allow for either kills, or a few assists early game.

Once you have Sheen you're good to go, Sheen is a key item to building Poppy, it gives her the necessary Mana she needs while allowing for a HUGE increasing in her burst damage. Essentially it's designed for a slam to activate and then your D-Blow to destroy their hp. Doing so out of a bush for surprise or to a glass-cannon character will drop their health so quickly few will stay to fight, allowing for an easy chase and kill.

The Madred's Razor is there so that Poppy has SOME farming capabilities. Farming for Poppy is rather rough without any item assist.

Banshee's Veil keeps Poppy from feeding mid/late game and makes for phenomenal survivability. Being able to resist Ammumu, Nunu, or Galio's ult, with potential to interrupt it with a wall slam makes Poppy very valuable in team fights. The added bonus of a health boost isn't too shabby either. Choosing between a Negatron or a Catalyst while building is up to the player depending on what the situation is with the other team.

After that your survivability is in check and it's time to become a killing machine, Poppy is a mid-late game Champ. Her ability to kill comes around this time, when she can survive getting past tanks to get to the squishy meat casting behind them. Trin-Force and Madred's BloodRazor give Poppy the ability to destroy both Carries and HP based Tanks. (Also,sell off your Philosopher's Stone at some point during these builds in order to both free up a space and get 400 Gold)

Most matches are well and over around this point, however I put some options that can happen after this. With the increase to attack speed that come with building a Trin-Force and MB Poppy's auto-attacks are fast enough to pick up some life style. Staying toe-to-toe with another champion and just burst damaging them and slamming them into a wall at this point is not out of the question.

Be sure to grab Heroic Charge first in 3's it's critical for that pre-minion spawn gank. And in 5's it'll incapacitate one target to the point where you and your lane partner can make quick work of them.

Waiting 'til level 2 however is best for the early kill, her damage output on Devastating Blow is just too amazing to pass up.

At level 6 she gets one of the greatest ultimates in the game! Her ability to ignore 4 (or 2) other champions damage while doing increased damage to your target. Keep in mind that while you're using Poppy's ult not even a turret can hurt you, but regardless of when you cast your ult, if that target dies so does your protection so be sure you have an exit route, whether it's away from other Champs or away from a turret. Also, sometimes it's best to not use the ultimate on the champion you're fighting. Casting the ult on a champion who is far or running away from you means that the champ you want dead can't even do damage to you, keep this in mind when selecting your targeting.

Rushing Devastating Blow is key with Poppy since all you want her carry for is a stun, her Q is where all her damage really is. Make that Q count and W and Ghost the hell out if you're in a bad place!

Ghost and Exhaust. The blind from masteries makes Exhaust clutch in a melee fight, and also allows you to hunt down your foes. Get a Ghost, Exhaust, and Paragon of Demacia off and nobody should be escaping you. Even only getting one or two of those off will still make it unlikely that they'll escape or that you won't.

Pros:

Her Ultimate allows for epic tower dives
It also allows for initiating fights
Her burst damage is incredible and with a little survivability she's able to get in and out of fights easily

Cons:

Poppy has a HUGE tendency to over extend, teammates that are unaware of your intentions might not be able to catch up with a badly placed slam. Leaving Poppy amongst champions that can pick her off while she's floating out in the nether.
Poppy has manna issues. Even with Sheen it is likely you'll have to go back at times with full or near full health only to replenish your manna.

Overall Poppy is a pretty powerful champ. Practice with her is huge because without properly timed, placed, and prepared stuns she is vulnerable to attacks. Her ability to stay alive in situations that would otherwise be impossible makes her clutch in a lot of scenarios. Be prepared to 4v1 a tower with her ult, or stick near a tower when low on health, would-be tower divers getting slammed into the tower when they make the attempt is all too hilarious as the tower picks them apart. Thanks for taking the time to check out my build and best of luck. Lemme know how it goes for you, good and bad.