Herco
I started using these picks when I first started playing bass guitar over 10 years ago. I have been hooked on these ever since. The sound you can get out of a bass with these picks is amazing, very bright and crisp!
The Medium thickness Herco picks are my favorite, but they ARE flexible enough that you lose a bit of volume when really trying to thrash some strings. The Extra Heavy picks are a bit uncomfortable over the top of the thumb, but are bearable.
I've played many shows and recorded multiple times with the Medium, Heavy, and Extra Heavy Herco picks. I can't play a show without them anymore! I've tried other brands of thumb picks (Dunlop) but these are simply the best!
These CAN and WILL break over time. I broke three Medium picks in one show, but I had been practicing and playing with them for three weeks. They're plenty durable, but will break eventually.
BUY THESE PICKS, they're GREAT!
PS- I've noticed people writing reviews on the other thicknesses of Herco picks saying that "They came stretched out, they don't stay on my thumb, etc." With hard enough playing NO pick will stay on your finger, and I always bend and flex the "thumb-ring" on these picks to fit my finger, if its too tight, I stretch it "flatter". If its too loose I "roll it under".
I still pinch this thumb pick between my thumb and index finger because I play a very rapid downstrum-upstrum (8th notes) style bass (punk/rock/alternative) but the thumb-ring on these gives you the extra support you need for those really fast riffs.