There are three good item builds for Poppy, Tank, AP, and Dps. I use the AP build the most because this build is the most OverPowered of all the 3 builds. The AP build deals the most damage with skills and got alot of runspeed and AP. The tank build is great for poppy because she can push an enemy away (Heroic Charge)from your allieds and and she can slow enemys with Frozen Mallet that makes poppy a great champ too save your teammates. The Dps build is the build that does the damage by normal attacks, for this build I use Frozen Mallet too because it gives you +700HP, +20Damage and you slow enemy champs with normal attacks.

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You can stun (Heroic Charge)
Ulti = OP
Pretty good lane pusher
Big chance for survival (passive)
Poppy is fast (Paragon of Demacia)
Good in 2 vs 1 fights (sometimes you can handle 3 vs 1)
Assasin (Backdoors)

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Not OverPowered and not UnderPowered in early game
Bad farmer

Valiant Fighter
Devastating Blow
Paragon of Demacia
Heroic Charge
Diplomatic Immunity

I have make my own combo of skills for poppy,

First I charge to a enemy with Heroic Charge(against a wall), next I use Devastating Blow that deals a lot of damage.
After Devastating Blow you have too
attack with normal attacks. When your cooldown of Devastating Blow is 0 use it again and again. When your Heroic Charge is out of cooldown you can push the enemy against the wall again and he is stunned again. If your enemy is still alife you have to repeat this again and again until he is dead. (tip after or before you've opened the battle use Paragon of Demacia )


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I use for the 3 item build difference summoner spells,

Tank
Flash: I use it for a quick escape, or too surprise the enemy
Fortify: I only use it as extra help and it increases your chances of winning
Revive: I use it because your team needs a tank so a long time without a tank can decide for winning or losing (if you die with a tank, tanks almost never die)

AP, DPS
Flash: I use it for a quick escape, or too surprise the enemy
Ghost: It gives you a speed boost when you chase an enemy or when your running away for the enemy
Ignite: it decides the life or death for the enemy (Use it on the begin of my combo to deal more damage)

About the items I have a little to say, for the tank build you have to decide between the Ninja Tabi or the Mercury's Treads because when the enemy have 3 DPS in team you have to take The Ninja Tabi when the enemy got 3 or more Magic champs you have to pick the Mercury's Treads.

This is my second build so its not perfect + English is my 3 language so there are alot of mistakes. If you have some hints or tips you can write it down below. I hope I will see you on the Field Of Justice.