League-of-legends – Is it better to take CDR runes on zed just flat A.D + armor pen

Well, for marks you should be running flat AD or Pen anyways and not CDR with out inefficient it is in that slot. The question becomes Do I want CDR Glyphs or MR? and also Do I want AD/PEN Quints or CDR Quints?.

Do I want CDR Glyphs? Depends on personal preference/matchup really. If they won't deal much magic damage or you think you can simply kill them instead of getting hit you will want these.

Do I wanat CDR Quints? This is the real difficult one. CDR efficiency in Quints/Glyphs is actually the same. Efficiency between AD/Pen Marks and Quints actually reduces when you move to quints. You would think that implies to get CDR Quints when looking at it that way but doing so might not be necessarily the correct choice. The more penetration you have, the most useful it gets because it brings you closer to doing true damage. Adding CDR gives you a little flexibility but the question is how much CDR do you really need with Zed as opposed to damage that lets you kill in a single rotation? You need to keep in mind that you will likely get a black cleaver which really gives all 3 of those stats at once. If you are building a black cleaver + Last Whisper, it makes the additional synergy of penetration even better since each point of penetration is almost twice as good since you will be ignoring about half of their armor already from shred + armor ignoring %. Really it comes down to preference/style. You can make plays easier with quints but do you get enough damage off by doing so?

TL;DR; On paper CDR seems more efficient. You add in items and the desire to assassinate as quickly as possible and then penetration + AD looks a little better. Late game Pen is definitely better but early game CDR can be more useful. It's honestly going to be personal preference but I favor lots of penetration.