For the most part I am now using 1/8" wall Buna-N rubber o-rings on my captive bit tools and tools that use o-rings for grip. Most common sizes are #202, 203 and 204. On my older tools I used thinner 3/32" wall sizes, mainly #109, 110 and 111 but I'm finding the heavier o-rings more to my liking.

Here's a link to the standard o-ring size chart:

O-Ring Size Chart