There is no one perfect answer to your question. Certainly, I have read very good user feedback on "Fedora Core 7" which offers very broad hardware compatibility as does the distribution, "PCLinuxOS 2007". For the purpose of simplicity, I will assume that you are addressing just the freeware versions of Linux.
For those who are primarily concerned about efficiency and stability, "openSUSE 10.2" may be the optimal choice.
However, if you are mostly interested in multimedia then "DreamLinux" may be your ticket to PC Nirvana.
For a Linux distro that's small, fast, and simple, you may want to check out "Vector Linux" or "Puppy Linux".
So one of the first things you need to do is decide what aspects of a distro is the most important for you.
The beautiful thing is that many of these distributions are now available in a LiveCD format so that you can test drive them without actually loading anything to your hard drive. This is great because it gives you the opportunity to check out and compare a number of different distros without actually installing any of them. Since there is no actual install, you don't have to setup a dual/multi boot option or uninstall your current Operating System. And it also gives you an indication of compatibility with your current system hardware. The following distros all feature a LiveCD option:
Ubuntu/Kubuntu 7.04 (Feisty Fawn)
DreamLinux 2.2 (based on MEPIS)
MEPIS Linux 6.5
PCLinuxOS 2007
Puppy Linux 2.17
Vector Linux 5.8
For info and download of the above you can use a Google search or go here: http://distrowatch.com
I chose to go with Ubuntu 7.04 using the Gnome interface (although the KDE interface [kubuntu] offers a more Windows like experience). The Gnome UI feels somewhat like a cross between the Apple OSX and Windows but the response is very snappy (as in quick).
One of the most compelling reason for me to stick with this distro is the fantastic support that you receive. User Community support is very patient and friendly and you are made to feel immediately welcomed as part of the community. The quality and timely manner of the responses are first rate. And the on-line Help sources are also excellent
The Ubuntu distro is one of the most popular and even comes preinstall on some of the new Dell computers. This translates into broad hardware compatability - I had no problem automatically accessing the internet through my established Windows broadband network.
Ubuntu also provides you with the low cost option of purchasing a licence to various protected, proprietary codecs which alleviates the hassle and legality of playing commercial audio [MP3] and video [DVD] ( see: http://www.cnr.com/press.html ).
Anyone that is thinking about investigating Linux, particularly current Windows users, should definitely consider giving Ubuntu a spin.