I think that are quite a few distros you can choose from.
I don't know Arch, but have seen Ubuntu and Debian.
For a user that doesn't know his way around I guess Ubuntu might be the easier choice.
In addition to those you may also consider Fedora or OpenSuse, which are also pretty easy to use.
Personally I like Fedora, because it is, in my opinion, the most cutting edge distro out there.
All of the distros you mentioned bring a lot of software (although I can't confirm this for Arch, but I just guess it does too), so you more or less just have to decide how recent you want the packages to be.
I like Debian, but for me it's not really a desktop-system, as it's already sort of outdated on release.