I was a fan of RedHat until they spun off their desktop distribution as Fedora last year. It's still a leading desktop distribution. I wanted something a little more hands-on and I've been using Gentoo for a year. It requires a little more work than most desktop distro's, but you learn a lot installing and maintaining it. Linux distro's tend to differ not so much in what software they use/support but in the philosophy of their developers. Take a look at distrowatch for distro rankings and summaries.
I've been a Red Hat user for several years, but recently decided to switch to Debian. I am fully converted and will never go back. Debian's package management system is much more powerful than Red Hat's.
I would suggest Suse or Debian. There a re a lot of debian based installations like Mepis ubuntu etc. Gentoo is too hard to install and if you fail to provide compile parameters properly, system will be slow.
If you are looking for a unix java environment, i would suggest Soalris 10 GA. JDS is quite good and updated versions of gnome, xfce and kde is available from blastwave.org
RedHat's philosophy behind their original Linux distribution was to be the leading desktop Linux distribution. It set the standard for ease of maintenance and stability. When RedHat spun off Fedora, they wanted Fedora to be a more bleeding-edge test platform for RedHat Enterprise Linux and that ment more maintenance and less stability. Now I may contradict myself when I say that I went with Gentoo as it is an even higher-maintenance distribution than Fedora but at the same time I was looking to learn more about how Linux works. Gentoo gives you options from using a bootable CD and installing precomipled packages (like Fedora) to compiling everything from the kernel up by hand. Open Source is supposed to be about freedom to choose and Gentoo makes the choices available far more apparent than Fedora. I'm not saying Fedora is bad, it's far more popular than Gentoo, it's just different.