Thumbpicks: those GoldenGate picks about 2mm thick. 'cause they fit my thumb best and last forever.

Flatpicks: toss-up

Thin Sharkfins by Landstrom (I've kept one for 8 years).
.54 mm large triangle immitation tortoise shell by Steve Clayton.

I hear that one thing Nancy Griffiths does is cut the end off a National thumb pick and then glue a thin flat pick in place. Might try that myself sometime...but with a GoldenGate.