I’ve used Slackware and Ubuntu. Both are great. I think these days Ubuntu is the “gold standard” for ease of installation.
I have it on a dual-boot machine with Windows 7.
The only fault I've found with it so far, is that it does not use the DVI output at boot time. In order to boot with other than the default OS, you need to use a VGA monitor to see the boot menu. Once the boot has completed, you can then switch your monitor to DVI. Since my monitor supports both VGA and DVI, I can just change a monitor setting to observe the boot menu. I do not know if this "problem" exists in Ubuntu and Debian also - I have not tested them on my other machine yet. It's related to GRUB - the bootloader which actually loads the operating system.
I would recommend either Ubuntu 10.4 LTS or Mint 9.
One more thing - if you want to have a dual-boot machine with Windows and Linux, ALWAYS install Windows first and then install Linux - most of the distros will do that from a single CD or DVD. On the other hand, a Windows installation destroys the Linux bootloader and provides no multiple boot capability, while GRUB is designed as a multiple OS bootloader.