Boxerbriefs. Not worn high up, but not low enough to show ass either - as someone mentioned. Dark colour, simple fabric is best in my opinion. Slightly but not excessively visible over your pant line when your shirt is off.
I cannot get enough of my guy in them (granted he's a hockey player, so he's natural pretty hot), he looks amazing. Today he was standing in the bedroom doorway for a while talking to me like that before going to take a shower and I couldn't help myself. I just got up and dragged him back to bed for another round. So, so, so hot. Boxerbriefs are really like the male underwear equivalent of victoria secret type looks for women.
Boxer briefs or just play out boxers are fine. As long as you make sure they come up to your hips like their supposed to. My little brother wears them (and he'll walk around the house in just that. He's like 12) and he wears them BELOW his hips. Making them show off his butt crack. Its gross. I'm always yelling at him "Pull up your underwear! No one wants to see that! "