In the case of men, I don't like slips, especially if it has the classic Superman logo, they look a bit childish IMHO.
Most Helpful Guy
I hate briefs. I haven't worn them since I was a kid. Not only are they extremely uncomfortable, but they look childish to me. Boxer briefs are better, and they look the best on me, but they still aren't as comfortable as regular boxers.
On the opposite gender, I hate seeing high-cut briefs and control briefs. I'm not a huge fan of regular briefs either. If she wants to wear something for comfort, she should go with bikini. However, I find boyshorts and cheekies most attractive. Thongs are OK, but like you said, they leave less to the imagination.
Most Helpful Girl
I dislike wearing boyshorts, I generally find them uncomfortable. I do find them attractive on other girls though. I don't think there's a type of underwear I find particularly unattractive on other women (except the really really thin g-strings look a little funny to me), but I really don't like briefs on men and I don't like boxer briefs much either.