This thing is still WIP. Thank you!

Unyielding : The 5% bonus armor and magic resist makes Poppy tankier earlier than opponents who don't have this.
Tough Skin : Gives you a little more durability against minion attacks, early harass, and DoT abilities from Talon or Teemo.
Runic Armor : Bigger shield for the buckler, more heals on Grasp of the Undying, and extra regeneration? Sign me up!
Fearless : Helps against burst mages and surprise ganks. The armor and magic resist you get from this mastery are very significant, and it comes up at least twice during a teamfight, so make sure you watch the cooldown.
Swiftness : Tenacity and Slow Resist on a tank? Yes please!
Grasp of the Undying: Personally, the most enjoyable mastery on Poppy. If you're farming, all you have to do is hit the enemy laner with your buckler, which triggers the effect. It's a great source of early sustain, and can put your enemies at a disadvantage early. Plus, it helps in duels, because the bonus damage and heal amount is deceiving.
Savagery: Helps to make farming early more forgiving if you misjudge a minion's health.
Assassin : Top lane is a lonely lane. Might as well hurt some people while you're there.
Merciless : More damage for low health targets makes Poppy a huge risk to stay around with low health.
Bandit : Hands-down, the most useful thing besides Grasp of the Undying. Your buckler counts as a melee attack, so it will grant you the 10 gold instead of 3 gold. Let's do this mathematically, if Poppy were to stay below level 7 until 20 minutes (which should not happen, mind you), she would get 75 buckler throws in that period of time. If she used all of them on an enemy champion as soon as it was ready, she would get 750 extra gold from the buckler alone. You'll get a little more than that by 20 minutes because the cooldown on the buckler becomes 13 seconds instead of 16 seconds, but I doubt you use the buckler every time anyways.

Offensive Items
Iceborn Gauntlet: A safe and solid pick for Poppy. While it provides tank stats, I prefer to use it as an enhancement for the Buckler.
Titanic Hydra: Don't build early, save it until you have around 2,500 HP.
Guardian Angel: A situational pick. Don't bother if you're behind, but it's essential if you're ahead.

Defensive Items
Zz'Rot Portal: One of my favorite items to buy on Poppy. The magic resist and armor in it are great, the MS boost helps with how sluggish Poppy is, and the portal itself helps push out a lane without even being there. You can catch your opponents off guard easily if they don't notice the portal.
Gargoyle Stoneplate: If you intend to help out other lanes, make this a priority. Otherwise, buy it after you finish your core build.
Warmog's Armor: Helpful if you have a lot of trades going on all across the map, or if your opponents have a lot of true or %-health damage.
Randuin's Omen: Helpful against Crit-based champions like Gangplank and Yasuo.
Thornmail: A good item if the enemy ADC is fed, if 2+ people on their team are auto-attack based, or if you plan to help out bot lane a lot. DO NOT BUILD FIRST.
Frozen Heart: A situational item for if you're against someone who has a lot of attack speed and AD. I generally only use it against champions like Jax and Kled, unless I have Thornmail. In which case, I will build it directly afterwards. Those two items in succession can easily cripple the fighting ability of an enemy ADC.
Adaptive Helm: A situational item for champions that have repetitive damage. Singed, Brand, and Teemo come to mind. Do not even try to build this item against Talon. Please.
Knight's Vow: A staple into the late game. Bind it to your ADC, and just watch as your opponents cry. Don't forget that you STILL HAVE TO FIGHT.

Vayne: A funny combination. Vayne shoots the enemy into the wall, Poppy helps to beat them up, then she uses her Heroic Charge as soon as the enemy recovers from stun. This stun timer puts Morgana to shame.
Jax: The stun chain here is great. Not as damaging as the Vayne combo, but still really damaging because he can do a lot of damage in a short amount of time.
Camille: Similar to the Vayne combo, except this method is much sneakier because Camille can jump over the wall.
Trundle: Instant wall? Yes please.
Anivia: Bigger instant wall? Yes please.
Taliyah: Even bigger instant wall? Yes please.