An inner or outer navel. I'd like to know which one you like the most around here and why. We can also put images as an example.

The inside of the navel is fine, but I definitely love one that shows more. Seeing a big pregnant belly with that busted navel is such a hot feeling... It certainly shows the great effects of pregnancy.

Belly button power rankings:
1. Flat
2. Outie
3. Innie

I really think it comes down to the individual's belly button. Some innie belly buttons are hot as hell, while some are not. I love both innies and outies. Belly buttons become a more sensitive erogenous zone during pregnancy, so navel foreplay is great during that time.

I prefer a flat belly button!

something like this photo.

As I say in such polls/questions of this nature - I know I'm very much in a minority on this one. My own personal preferences, if you don't want all the detail, would be:

1) Innie
2) Flat (a very close, second...)
3) Outie

If you would like a little more of an explanation, I've never been the biggest of fans of outies. I'm okay with a 'slight outie'. The examples which I don't particularly like and to an extent find off-putting are the larger ones, which stick out wholly by, for example, half an inch or more. To me, they just look more like a 'lumpen mess' than anything else. They just don't do anything for me personally. There's even been examples in the past where I've found myself...'disappointed'...if that's the right word, where I've followed a pregnancy progression of an attractive belly button, only for it to completely pop out like this around the six/seven month mark... 

Innies and those which are flat or close to being flat have to be my own personal favourites. As I've described several times in the past, the belly button to me is the 'face' of the belly. The ideal combination of a navel and pregnancy fetish, for me, is to see the belly expansion and the best way to mark it would be to see the 'innie' slowly stretch, deform and shallow out. A way of really showing the increasing pressure and size it's having to deal with. Ideally, by the end of the pregnancy there's not much, if any, 'indent' of a navel left. Maybe some part of it is slightly raised above the belly's surface, but not completely popped out... In my own eyes and for my own tastes, it's these sorts which are the ideal pregnant belly buttons.