Dark-colored boxer briefs are definitely the sexiest, but from guys I've talked to, aren't always the most comfortable.
I used to think that boxers on guys were the standard, and I would just have to get used to how awkwardly short and funny they can look.
One fellow whom I am quite fond of is especially well-endowed, and boxers just don't, erm, support him enough, and are uncomfortable. He wears dark-colored or black low-rise briefs (definitely not tidy whities!) and they are incredibly sexy. When we first talked about underwear (hey, it happens!) I was surprised that someone around my age would wear briefs, but nowadays, I am quite a believer in them. They seem a bit classier. Bit more... European, maybe, and less college frat boy? Haha.