Well, I am so much in love with the lower part of a girl's belly (including the navel of course), that for me the "actual" belly is the lower belly.
If the lower part is not fully exposed I feel like there is no belly exposure at all. For that reason, my ideal outfit is one that covers the upper part of the belly and fully exposes the lower part.
A top that stops right at the navel combined with a super low pair of jeans would be a very good example of what I prefer.
I think Lindsey Vonn looks fantastic in that outfit. I also love the fact that her navel is partially hidden but when she moves a little bit it shows. That's devastatingly beautiful, isn't it?