I use Eclipse and it provides an editor for Ant: syntax highlighting, command completion, keyword suggestion, etc. I suspect that NetBeans, IntelliJ IDEA, and other Java IDEs also provide this capability.
Notepad++ will give you syntax highlighting and with the XML addon will get you block completion (example: you type "<copy>" and it will fill in the "</copy>"). And it lets you create code snippets (such as the <copy> or <target> blocks. But you won't get the auto suggestion and auto complete that IDEs will give you.