I was going to suggest
Knoppix which is what Morphix is based off of. The great thing about Knoppix and Morphix is that you don't need to commit a partition to install Linux. Your hard drive is untouched as everything runs off the CD. It's a great way to get your feet wet if you're just starting out with Linux.
Once you're comfortable with the basics and are willing to at least dual-boot, I would suggest
Gentoo. Not only does it have a great package management system that rivals
Debian's APT, but you can completely customize it to your heart's content. The only drawback is that everything is built on the fly from source tarballs, which means if you have a slow system big packages can take days. On the other hand, there's nothing more satisfying than a wildly tweaked-out and hacked kernel.
Debian still has a place in my servers. There's nothing more rock-solid than the Debian stable tree.
Unless you're looking for a business desktop, I'd stay away from RedHat and even Mandrake. Installation is easy enough for any Linux newbie, but the end result is lacking for any Amigan and the RPM package management system (even Mandrake's URPMI) can leave you frustrated.