I'm not a "boob guy", but underboob is fantastic. I'd rank side boob up there too.
I think cleavage and the top of the boob get all the love because under boob and side boob are ridiculously hard to pull off without exposing too much, and you can't have a mediocre rack...
"Happy slaves are the worst enemies of freedom." - Marie Von Ebner
"It was always the women, and above all the young ones, who were the most bigoted adherents of the Party, the swallowers of slogans, the amateur spies..." - Orwell
Underboob is the internet musician of boob. You got to be good to be a successful internet musician; you can't hide your boob flaws behind push-up marketing and frilly bits of public relations when it's just raw like that.
That's why it gets underrated. Most women can't hack it.
"ATTENTION: Customers browsing porn must hold magazines with both hands at all times!"