Try netbeans www.netbeans.org

To understand how it really works - notepad/vi.

Define "best". I use Netbeans. Reasonable people can and will differ.

http://www.netbeans.org/

The best is obviously Eclipse.

http://www.eclipse.org/

Regards,
Wald

For Unix/Linux users or people used to the way open source stuff works -
Eclipse or NetBeans.

For people like me who use, and like, Windows programs, either JCreator or
JBuilder.

I personally found Eclipse impossible to work with and totally non-intuitive.
Other people love it.

My opinion only - YMMV.

For starters? I'd use Notepad (of VI, or Emacs) and the command line.
Make sure you can write, understand, compile and run a simple "Hello
World" application.

Then, try a few adn see what you like. My personal favorites are
Eclipse (free) and JetBrains Idea (commercial).

--
now with more cowbell

For learning:
notepad, textpad, JGrasp

You'll hear a lot of people talk about vi/notepad and javac.  Then you'll
see a couple with some IDE.  The pros/cons are:

editor and javac
pros:
-you'll learn a lot about packages, command line tools, environment
settings, batch files
-you'll know a lot about the java lanaguage
cons:
-takes a long time to actually learn the language cause you're learning it
truely from first principles
-can be frustrating as you may spend quite a bit of time getting things in a
working state before you can get down to the fun and interesting stuff
(swing, inheritence, etc)

IDE
pros:
-gets you up and running quickly
-lets you focus on learning the language
cons:
-very easy to become over reliant on the IDEs enhancements/tricks/etc (code
insight/intellisense, the gui designers)
-you tend to want to learn the IDE better instead of the language

This discussion comes up every once in a while, and it truely comes down to
what kind of student you are.  If you bore easily, then maybe the IDE is the
better way to do as you can get to a running temperature conversion program
quickly.  The HelloWorld is too easy with either method, after you do this
example, is where IMHO both methods start to show their differences.

The one thing I don't believe any posters have mentioned is creating
bookmarks/favourites to a few key sights to help you out.  As soon as you
get into trouble with something start learning/reading up on these sights.
API docs: http://java.sun.com/j2se/1.4.2/docs/api/index.html
java tutorial: http://java.sun.com/docs/books/tutorial/search.html
and the best site of all for help?
http://www.google.ca/

Good luck!