I've reached a point now where I can use pretty much anything, including coins, and get reasonable results. Just try whatever takes your fancy. Eventually you'll find what works best for you.

Nobody can tell you what pick you should use because nobody has your fingers, wrist, strumming/picking motion etc, etc...

My absolute favourite pick is 1.2mm and made of sterling silver (a gift from my son), which has absolutely zero flexibility, but mostly use a weird piece of 1mm ish thickness pick-shaped plastic I found in a box of electronic components or a .60 Dunlop Tortex (also, because they're bright orange I never lose them - apart from one that fell into the sound hole and stuck somewhere in the struts, although technically it's not lost as I know where it is).