Hey and welcome to my Poppy build. I think Poppy is a really good hero with a lot of damage. She's a really good turret diver with her ultimate and can really initate team fights.

This is the items i think are the best with Poppy, effect from Sheen is really good early game.
Poppy is best as hybrid, and i go for Infinity Edge before rabadon since AP is more important late game.
Poppy is kinda squishy without her ulti ready and in long games really need a survival item.
If u are fighting a lot of AP heroes like Veigar, Ryze, Annie and more u should buy Banshee's Veil. But if their team is a lot of AD heroes like Master Yi, Xin Zhao, Pantheon u should buy a thornmail.

Those runes are not very powerful late game, but gives you great chances to become feed early game since ur Devastating Blow will deal really much damage early game with those runes. This is just what i like and want when i play, dunno if its really the best.

Flash is great for pusing people into turrets doing like this, flash soo you are standing behind them, then use Paragon of Demacia followed by a Heroic Charge. If ur doing right, u will be able to push them into the wall next turret.

Ignite is a good spell for most AP heroes, since if u specc it gives 10 AP while on cooldown, and that is really helpful early game!