Simply put, there's not one.

Choice of a distribution is entirely subjective to the individual's needs, skills, and crap tolerance factor. I'll put some of my opinions here - you're free to agree or disagree, but please don't turn this into a Jihad, or XP pack rape if someone says something you don't agree with.

  • Debian - considered by many to be the most powerful distro, I prefer it for a server because it's generally more stable than other distros. Has a kick ass packaging system, but the package manager dselect can be confusing at times for the uninitiated. If you're sick of Redhat getting in your way with it's gui crap, then this is the distro for you.
  • Redhat - generally hailed as the distro that brought linux to the masses. A decent distro, it's what I'm currently using as I write this. If you want ease-of-install and pretty gui stuff, use this. I don't neccesarily like the gui stuff, but it's a simple distro to install and maintain, which is what I want in a workstation. If you're new to linux, I recommend this. It's popular, so support is easy to find, and the Redhat Package Manager, while not being the most powerful package manager, is pretty easy to handle.
  • StormLinux - I've not used this much, but from looking at the install it looks like linux for your mom. Simple, easy. It's relatively new, however, so it may still have outstanding bugs. Caveat emptor.
  • Mandrake - A simple distro, it combines the ease of use of Redhat with a friendlier, more touchy-feely feel. Upgrading from one release to the next can be problematic, however.
  • Slackware - often referred to as 'Snackware', this distro always leaves you wanting more. I haven't used any recent releases, but the older ones all sucked donkey nuts. No package management worth speaking of, a metric fuckload of security holes out of the box, and a shitty installer. I considered this a real fixer-upper distro, for hardcore masochists only. I hear it's better now, though. Just don't use any old releases, or you'll be 0wned relatively quickly.

While this may not be an all-inclusive list, that's what the big box at the bottom of this page is for. If I've made a mistake, or left out a distro that is ohsogroovy, let me know.
After reading the comments below, I felt I needed to clarify my statements.
I was down on Slackware specifically for the reasons you mentioned. I used to think that knowing my shit about Linux was where it was at, and it was, when I first started, but now I just want something that works, works good, and works fast. Something I don't have to fuck with to make it work. Of course, Redhat and friends still require a fair amount of gyrations, but much less than Slack. And the new Slack may whoop ass, I haven't looked at it. And yes, I've used old Slack, and old Redhat. I've even used Yggdrasil, but that's been a long time.
And as for the KDE/Gnome war, fsck em both. Blackbox 0wnz j00. :)