Well, this is my first build ever and I just published so i can recieve some feedback. I'll just try to explain quickly why i chose everything. This build is intended to be flexible. that's why i didn't mention if it is for top, bot, mid or jungle.
By the way, if you think you can improve this build even a lil, or just want to say it sucks (obviously it would be better if you tell me why). Thanks for your time

Here's a pic with the real stats when you complete the build

why so many runes for AP?

well, i think that the quicker your target dies the better. I want to appear and before the enemy can think what to do get the kill and get the hell out of there. So that's why i chose Greater Glyph of Scaling Ability Power, Greater Quintessence of Scaling Ability Power and 3 Greater Seal of Scaling Ability Power. Then Greater Mark of Magic Penetration so all my AP can go somewhere and 6 Greater Seal of Scaling Magic Resist to protect me a little

As you can see i'm trying to do the same as with the runes, get as much AP as possible while considering a bit of survivability. Trying to lower my cooldowns and get more mana regen to attack faster. More health, spell vamp, armor and magic resist to keep longer in battle when needed.

If i explain this i'll be saying the same as in runes and masteries and i'm sure you already got my point. But...

Why is there a Hextech Revolver?

I take the Hextech Revolver because of the spell vamp, 12% of this can really make a difference between life and death. Actually i wanted to include Hextech Gunblade in the build but at the end it didn't get me to 500 AP

Why i wanted to get to 500 AP?

Because Neurotoxin / Venomous Bite adds 2% of the bonus damage for every 100 points. And with Hextech Gunblade it was close to 500, so i said. why not get to it?

I do take Hextech Gunblade if i see i die too easily but, if the enemy team can handle my damage output, i don't need spell vamp anymore.

Ok, this is the good one. Let's check the questions you'll probably have

Why did you hand out the first 3 skill points in that sequence?

I take the first point to Volatile Spiderling / Skittering Frenzy because in human form it hits multiple targets and i can make it hit from behind the minion waves and chase my opponent when he tries to get away. And in spider form it helps me clear quickly minions or monsters and getting some health back.
Then Neurotoxin / Venomous Bite For that 8% of current/missing health as bonus damage to kill my target.
finally, "Cocoon / Rappel" to trap my enemy in human form so i can run into a battle, and chase him down in spider form to make sure there's no escape.

Why did you max out "Neurotoxin / Venomous Bite"?

Low cooldown, awesome damage output. DAMAGE BASED ON HEALTH! i think it is quite obvious why this goes first and Cocoon / Rappel goes last. I mean, it only reduces the cooldown! maybe the story would be different if it also increased the stun time and increased the range of rappel (it is quite big so it's not needed, but can you imagine how OP it would be?).

Get a point first with Neurotoxin / Venomous Bite instead of Volatile Spiderling if you're top or bot. You won't fight before minions spawn at mid (at least, i haven't been in that kind of situation) and to deal twice the damage in the first battle can be the difference between life or death.

I think you know why i take Flash and Ignite don't you? just switch to Smite if you're going to jungle.