Let's face it; the underwear you choose at the start of each day will play a major part in how your day plays out, so it's best to go with what's most comfortable.
For me, it's this brand called "George". I had no idea where they came from, as an ex bought them for me when we were going out, but OHMIGOD they're comfy! They feel like cotton and have a slightly stretchy quality, but remain slim-fitting and don't have any of that "stretch-out" feeling that most boxer-briefs seem to have at the end of the day.
Any other worthy candidates?