Adam, hands down. The curve, the size and the feel are all excellent. I have only tried one other O2, the Cush, which is also nice, but I prefer Adam. (This isn't reflected in my reviews of each, however, as it took me a while to warm up to Adam.)
The Adam is the one that's been lingering on my wishlist for a while. (Just as an FYI, the VIP Supersoft is NOT dual density. It's the soft squishy stuff all the way through. Just to warn you before you think about ordering it. )
I have Mark, Adam, and Flurry. I never use the Flurry because its shaft is so straight and textureless that it's hardly stimulating at all. I use Adam occasionally; it has an awesome G-spot curve, but it's a bit too girthy for me to use on a regular basis. Mark is the one I reach for the most, because it's the perfect size for me and has a small curve that works fine.