Q:

What Runes should I prioritize buying?

I primarily play Ranged AD, various Junglers, or solo top champions. Looking for which runes are the most universally useful/most cost effective to buy across many roles. These are the sets of runes I own.
Marks:

Flat AD
Armor Pen
Magic Pen
Attack Speed

Seals:

Flat Armor
Mana Regen/Level

Glyphs:

Flat Magic Resist
Flat Cooldown Reduction

Quints:

Flat HP
Flat AD
Movement Speed

A:

I found this image flow chart to be rather useful for getting up and running with runes. These are general idea runes and once you get to know some champs you may want to get something different but I feel like this is a good start.

A:

First and foremost I'd get some scaling magic resist glyphs. The flat ones are great when you're mid, or you know you're going against a mage up top (e.g. vlad) but for blind pick or bottom lane or jungle I'd always go with scaling over flat.
Other than that, you have just about any runes you'd want to cover those roles.
I'd recommend picking up a mage page (flat AP quints, flat AP yellows, magic pen reds) and a support page (avarice quints and yellows, flat armor reds); being able to play a variety of roles is more important than being specialized at one IMO.

