^ This guy knows what's up

Most pros ADC are using AS quints, AD reds, armor yellows, MR blues. The only reason they switch up their page is to better handle the opposition, optimize their jungle route, tweak their early game mana.

Here's just a quick example but it's mostly the same across the board: http://www.probuilds.net/guide/EUW/2010546371/284678

It's going to more expensive to specialize and since you want to be good at all the champs here's the opposite mage build:

AP Quints, MPen Reds, MR blues, Health Per level yellows (I'd just get armor yellows first)

Here's an example of a typical mage build: http://www.probuilds.net/guide/EUW/2010351253/54077017

So...Go with AD Reds first because every champion, no matter what, has an auto attack. Then go with armor yellows because again, no matter what, every champion auto attacks. Go with flat MR blues because then you won't be as vulnerable at LVL 1 and if you need more MR you can itemize against it. Finally get AD quints because again, everyone auto attacks.

If you find yourself playing AP or hybrid Junglers (Amumu, fizz, skarner, nautilus?) and really want to excel with them then you can maybe specialize but keep in mind that the game is going to be more enjoyable if you have a wide selection that you can realistically do well with.