It wont really matter, as far as 'prettiness' goes, what distro you are running.
For example, KDE 4.6 can be made to look the same on any distro. No matter if you are using arch, debian, mandriva, slackware, etc..
The 'stock' versions will look different. But once you get into the settings and change a few things around, you will find that pretty much any DE/WM (desktop enviroment/window manager) can be make to look like the same DE/WM running on a different distro.
If you are really interested in appearance, getting a good idea of what DE or WM suits you, then learning how to change the settings to reflect what you find attractive is a far better idea than asking a forum what the 'prettiest' distro is.
Originally Posted by ongte
"pretty" is pretty subjective. Everyone will tell you something different.
If you asked 'what is the is the prettiest car ever, who is the prettiest hollywood actress, which is the prettiest timber', you will get different answers. If somebody says 'I think the Ferrari 250 Testa Rossa is ther prettiest car ever', that is an expression of personal taste, not some hard and fast fact.
I've always found gnome to be as ugly as hell. Running compiz (or similar) just makes it a flashier kind of ugly. IMO. Half this thread would probably disagree with me there (and its possible that a few of them arent ubuntu users LOL, though most would be).