On themselves, anything that labels their penis in a humorous fashion. Failing that, anything that's been cleaned sometime this week. Failing that, pretty much anything. Nothing is also fine.
When it comes to women, our standards immediately become much more exacting (although the "nothing is also fine" rule still holds). Since tastes differ, I can't tell you exactly what your guy will be looking for, but I can share that in my experience, women tend to think we want a lot of lace and connectors and frills and things that truss up various bits.
We do not.
In fact, anything that requires more than one step to remove from you is probably more exciting for you than for us. Which is fine...for modeling in the mirror or taking some racy photos to text us. If we're actually going to be doing the nasty, we'd prefer underwear that strike a balance between elegance, class, hotness, and removability.
Personally, I like those cute little boyshorts you guys seem to always have around. And a thong is classic. But each guy's going to differ. If he's up for it, take your man underwear shopping with you (keep it under half an hour, please), or ask him what he'd like to see you in.