Hi Marty. Hate to be the first one here to tell you, but Merle never USED a flatpick!
Good question though, and once again you'll get a variety of answers...all correct, for the folks that "do it that way". Bluebelle's obviously experienced, and has some good info, but he may not be aware that "Clayton Golds" are hard to find in many areas. Dunlop nylons are usually found everywhere. Tortex, as well.
My advice is just as subjective as anyone's. I've been flatpicking for many years and have changed brands and thicknesses more than I've changed underwear (well, almost). To complicate matters I have "outdoors", "indoors", "rhythm" and "lead" picks, cause I simply haven't found one that "does it all". I like a lot of "dynamics" in my playing, and for that I use slightly lighter picks than if I was only playing single line leads. Tortex '73s seem to be the best "compromise" pick for me. (Like Willie) If you can find them you might try "Reefer picks". Once again, these are regional and not often found, but who knows? YOUR region may have 'em. They're very thin, but very stiff. I bought a lot of them a few years ago, and save them for special occasions.....the problem? They break after three songs!! Turtles don't break......unless we humans do the breakin'.
A couple of things to keep in mind though..John's reminder about using the "round edge" (I don't, but I FILE the point round) and Midchuck's "hole" for easier handling if you're new at it, or simply like it that way.