Very subjective! There are loads of IDEs out there, personally I use Eclipse and I really like it. Even better for Mesham all I had to do was write a plugin and it is an IDE for that language also

The best thing about it is the history feature, where you can compare code to what you had weeks ago (and go back if need be!) Also things like autocomplete, syntax colouring etc are very useful.
There are many other IDEs out there - NetBeans, BlueJ, RealJ spring to mind. A google search should easily find them for download.
Let us know which one you choose and why you like it

Nick