I am basically using Aeon, the default, for both the main site and the forum. During development I used Nexus quite a bit, and for a while also Marble, but I find Aeon looks better on all types of setups (which is why I chose it as default): from large/hi-res screens to netbooks and the like, whereas Nexus looks really ugly on anything but a large/hi-res monitor, and the others also need some space to look their best.
One thing to note about the main site's style: If you try to switch back and forth between them on the fly you'll end up seeing some pages in the new style you picked, and others in the previous one. The way to do is like this:
1. Decide which style you'd like to use
2. Clear your browser's cache
3. Pick the style
Then use it for as long as you want, and then, when you decide to switch again, follow once more the above procedure.
The forum style, on the other hand, can be switched on the fly with no problems.