check out this link, its my favorites guide, created by win10cent

I go with :
red: 6 armor pen + 3 atk
yellow: 9 armor
blue: 9 MR flat or 9 MR/lv
Q : 3 flat ad damage

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with materies, its will give you 16 + 10% armor pen = true damage if enemy don't have any armor rune , with Q damage its still good at early
, I tested a lot of time, about full ad rune or full armor pen rune, its difference...abit
combo armor pen + ad rune will give you a huge of damage

just check out this link, this guy win10cent make a lot of calutalor about ad or armor pen rune:
http://leaguecraft.com/strategies/guide/11826-a-very-detailed-ezreal-guide.xhtml

just don't use too much mystic shot to kill the minion, try to last hit by auto atk ,and save your ***ing mana to hit the enemy, because we don't have mana rengen rune, and ez is mana burner, then I put 7 point into utility tree for mana regen
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next I put 5 point for CD and magic pen, don't tell me its waste, no... pls, 3 skill AP : W + E + R will deal more damage than you can think, and 4% CD is nothing, but its ok

start with doran blade if you lane with healer sp, if not, go with boots
after all, doran + shoes + vamp ... go BF > BT
go next is IE or PD, which first ?? your choise

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if 3 of enemy have > 120 armor, go last wishper, as soon as if possible

===some last item: GA / Qss / banshee / warmog / frozen mallet

depend on their team and what you need.

1. flash + ignite: not bad
2. exhaust + ignite: if their team have 2 carry or your team need it
3. heal + ignite: its new style now
4. flash + clean : got *** alot
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without flash, just remember use Arcane shift .... right people, right place, right time......or not, you will kill your ***ing self