What are you wearing right now that you like? I've used Smartwool undies and some Calvin Klein 100% nylon boxer briefs, but never for really long trips.
I've got some Icebreaker 'Zone' shorts. I'm a huge fan of merino anywhere I can use it. I'm curious of the supposed benefits of the 2XU type shorts for recovery, reduced fatigue, blah blah etc... My legs were so swollen after the AZT300 last year and at north of $100/pair was curious if anyone has used them (at all or) for multi days.