I am a big fan of his socks as well, but heard he got into the clothing business because he couldn't find underwear that fit well. IMO, he got it right.
For undershirts, my favorite hands down are the higher end 2(x)ist Pima cotton. They are a slightly ribbed design that fit close to the body but aren't at all restrictive. Their regular line is nothing special. Not a fan of their boxers.
I like these so well that I have gotten rid of all my other underwear now and these are all I wear.