I don't like to talk or explain much, but I will explain as much as a can.

Magic Penetration Marks to shred through those annoying tanks and destroy them. Armor Seals because you have a low base armor and this is important against AD midlaners. I take Scaling Magic Resist Glyphs, because you need it so you don't get easily killed, and you are a very squishy champion. Lastly I take AP Quints, because Ziggs needs a lot of AP and destroy his enemies.

Many people use Thunderlord's Decree on Ziggs which is a good option, but this keystone is mostly for one-target champions. I take Deathfire touch on Ziggs, because all of his abilities are AoE (Area of Effect), so let's say for example you throw your bouncing bomb on 3 enemies and they all get hit, they will all be affected by this, so it's very great in teamfights.

For Ziggs you can just take Ignite and Flash as almost any mid laner would. If you are against a burst assassin like Zed, I recommend taking Exhaust and Flash, and If you are against an annoying super ranged champion like Vel'Koz/Brand I would recommend taking Barrier and Flash.

Pros:
All your abilities are AoE abilities.
You have a lot of damage.
You can engage in teamfights with your W.
Your R is the best teamfight ability.
You can constantly poke your enemies with your Q.
You have a great escape with your W and E.

Cons:
You are very squishy.
You are weak against super ranged champions.
You will be the first target in teamfights if you aren't at the back of the fight.
No escape when stunned/snared.
No shield/protection.
You will have to ward a lot since you are squishy and will get 2v1'd a lot.

Ziggs can farm very fast due to all of his AoE abilities which destroys minions easy.