Thumbpicks are used by banjoists, guitarists, dobro players, other stringed instrumentalists to get a brighter, clearer tone and make it easier to play.
There are 3 main factors to look for when choosing a thumbpick.
Thumbpicks are made in 3 general styles, with a lot of variations in between. They can be made of:
Metal wrap and a plastic pick
All metal - the wrap and the pick
For me personally, I find the thumbpicks with a metal wrap to be the most comfortable.
ProPik thumbpicks all have a metal wrap and they are good because you can bend the metal to fit your thumb perfectly.
BlueChip Thumbpicks are the winner for me in this category. You can't really bend the metal wrap of these picks, but the wrap just seems to fit my thumb perfectly. I could wear this all day without it annoying me.
Plastic wrap thumbpicks can be comfortable, but they are hard to resize if they don't fit quite right. Some people put them in lightly boiling water to then make the plastic malleable so that the player can bend them to fit their thumb.
Most players use a thumbpick with a plastic pick on it to achieve the best tone. Almost all banjo players do. Some guitarists do like the thumbpicks with a metal pick on it. A metal pick will give a brighter tone.
When choosing a thumbpick with a plastic pick, the different type of plastic used makes a big difference in tone. In this category I would definitely put BlueChip first. The plastic used is a very high end type of plastic that is very hard a produces an incredible tone. Next up would be ProPiks. The plastic used here is a step up from standard picks and is pretty hard and durable.
Here are my top 3 listed in order of preference of thumbpicks for their tone:
Last but not least is playability. What's the use of a comfortable pick with great tone if it makes it hard to play?
In this section there are many different variations. Sometimes the pick part is very large and other times is is much smaller. ProPik makes a Quickpick that has a small tougue like pick on it that some players find it easier to play fast phrases. There are also thumbpicks where the the pick is in the style of a guitar flatpick but attached to a wrap to hold it onto the thumb.
This is really something that you just have to keep trying different things and see what works for you. For myself, BlueChip again takes 1st place.