Guitar pick thickness?

Medium or heavy.

If you use picks that are too light, you lose the ability to articulate anything with your pick and end up just kind of flailing in the general direction of the strings.

I like Dunlop Green Tortex picks.

Personally, I never use light picks, as they wear down & break easily. Pick thicknesses are usually just whatever you think is right, but your strings an affect it as well. I play Rhythm and occasionally lead, and use "BRIGHT Round Wound D'Addario XL Nickel Wound Regular Light Gage" strings, and I'm fine using fender medium picks. For like really fast, hard strumming, you might want a lighter pick that glides. For intricate picking, use Thicker, Heavy picks.