The best undies are none at all, which is how I always wear jeans. But, in dress pants I do need something underneath. I prefer the style A083 cotton Mesh Contour Briefs from Undergear/International Male. They're soft and stretchy, and fitted nicely in front so as not to constrict too much. They have a thin, soft waistband. I hang a bit heavy in them, which causes the waistband to pull down a bit in the front, but this doesn't cause them to "ride down." About $10 regular price, but they're often on sale for $8 or less.

http://www.undergear.com/HanoverAsse...ges/a083zz.jpg