From my years in linux, Mandrake is the easiest to use. Suse and Red Hat are coming up a close second now. I started with Mandrake and it made for an easy transition. However, if you are comfortable diving in a little deeper to begin with I would not recommend any of these.
Like I said, these are the easiest to use, i.e. they have wizards and central controls for just about everything. Because of this though, they are not always the easiest to learn the underneath of. Grab Slack or gentoo (I prefer Gentoo) for a good distro that lets, well, requires you to lean not only how to use it, but also how to understand it. These also provide by default a system that is a little more accepting of lower level tweaks and customization.
Well that is my 2 cents. I personally am still on Mandrake. As soon as my connection increases (DSL) Gentoo will be a quick replacement!