For the title of easiest Linux distribution for a beginner I would suggest Linux Mint.
Linux Mint is actually based on Ubuntu Linux, but simplified for users coming from Windows. The user interface has been tweaked to look and behave more like Windows. Most common applications like the flash pligina as well as video and audio decoders come pre-installed with the base distribution, meaning less things to configure for the beginner user. Linux Mint has a very polished software installer where hundreds of applications can be downloaded and installed in one click. You can also view a screenshot of the application that you are about install so that you can see what the application is about. Furthermore all free Ubuntu applications can also be installed on Linux Mint thanks to another installation utility: the package manager. On the downside there is no possibility to buy paid support and no application store for commercial applications. A great advantage for users that come from the Windows world is that thanks to Wubi you can install Linux Mint alongside Windows: it is like installing a Windows application. Upgrading Linux Mint to the next version is less intuitive than with Ubuntu, a design decision to ensure that the user understand that there is always a risk of problems during upgrades. This is not an issue if you intend to stick with your first Linux installation for a long time, but can be a problem if you want to upgrade your software from time to time. Should you run into troubles the community is great for providing help but is smaller than the one of Ubuntu.
To get a more in-depth look at Linux Mint i suggest that you check out my review on Tech-no-media.
The second best Linux distribution for beginners is Ubuntu Linux, even if it is not the easiest.
Even if it's not the easiest distribution Ubuntu is still easy to install and configure for the beginner. Like Mint you can even install it like an application in Windows. Once installed you can easily add more applications from within the add remove programs applet of Ubuntu because this distribution has the best selection of native Linux applications available. When a new release of Ubuntu is available it is easy to upgrade to the new release thanks to the Ubuntu upgrade manager, although this is sometimes risky. Because the community is large you can also easily get support and advice should you encounter a problem, the Ubuntu forums are in my opinion the best of any distribution for beginners seeking help. If you want to buy commercial software like PowerDVD or some professional phone support these are available through Canonical, the company behind Ubuntu. In the disadvantage column there is the fact that some common applications like adobe flash or video codecs must be installed manually, although it is easy to do thanks to the restricted extras package. Another thing that makes Ubuntu less desirable is that the user interface is somewhat different from Windows. Don't get me wrong, it is an excellent user interface, but for beginners this may be disturbing. Despite this I still consider Ubuntu as one of the best Linux distribution for beginners.
To conclude I would say this: the easiest Linux distribution to install and use for beginners is Linux Mint, however if you want professional support or commercial applications the best Linux distribution for a beginner is Ubuntu. If you want an easy install "just to try Linux" you should use Linux Mint.