Guitar Player magazine years ago did a whole-page photo of the picks of 50 top guitar players. They were wildly different.
That's what you'll find...Ask 100 players, get a hundred answers.

Also...By "flatpicking", most people mean playing in the bluegrass style with the "boom-chicka" rhythm and also fast, single-note lines and solos on big, stout-stringed dreadnaught guitars.
Heavy picks are required....

Picks are cheap; take a few bucks to the guitar store and buy a handful. Experiment.