Sajan we use Mandriva Linux at work for our workstations it is easy to setup and cheap. Yet if you're new and just want to test get Mepis or Knoppix Live CDs and check them out. You can run them without even touching your install so you can get a feel of it.
"Best" is a word frequently misused by the ignorant. Which is "best", apples or oranges? Depends on your tastes, needs and even allergies.
In this case, there's also a number of other factors. Linux doesn't actually have a GUI. Linux is the kernel, and if you say "Linux", meaning "gui" without saying "gnu", Richard Stallman will come and batter you about the head and shoulders.
Linux GUIs are based on a windowing system (almost invariably MIT's "X" window system) and made fit for human consumption by the addition of a desktop. Most distros default to either KDE or Gnome, though there are literally dozens of other desktops that have been developed for X. For the big name distros, like Red Hat, you generally can either select which desktop you wish to run under and/or can switch to a different desktop via a simple user command.
A lot of the config utilities and such have gui wrappers. These are programs, often in Python (Red Hat likes Python for this stuff). All they do is present dialogs, take the data and invoke the command-line config utility . Historically, these apps would use curses to make a character-mode gui (using "curses") if you weren't running X, otherwise they'd present a gui dialog(s).
With all those options, the word "best" seems to be a little feeble, I think.
The reality is, pick whatever makes you comfortable. Generally, that means whatever people around you like, since that way you'll have more local support. In the US, Red Hat (Fedora, if you want "bleeding-edge" stuff) and SuSe are preferred for business due to their vendor support, Debian and its spinoffs (including Knoppix and Ubuntu) are popular with home users. Other countries often have their own preferences, as there are many regionalized variants.
Java's not all that picky. Almost any Linux will run Java, as long as the hardware's up to it.
An IDE is no substitute for an Intelligent Developer.
I would recommend Ubuntu linux as an introduction. You can try out a "live" version on your PC which doesn't install anything, but lets you see how Ubuntu would run.
Not sure of your hardware setup, but for me I have an IBM THinkpad laptop with a funky ATI video card, wireless network adapter etc. Ubuntu managed to recognise all of the hardware on the laptop, so the installation was a breeze.