It all depends on the development framework you are working with, and the (financial) means you have:
>> If you are primarily a .NET developer it's a no-brainer; Visual Studio all the way (if you don't have a version yet, go for 2008 right away).
>> Another great IDE in my opinion is Adobe CS4 (for general Web/Flash development and graphic design), but Adobe products can cost you an arm and a leg, and take FOREVER to install. The cheaper (cheap as in FREE) alternative is Adobe AIR, which is a good tool for AJAX/Flex/Flash developers.
>> The last suggestion I have is Notepad++ (also free). It's pretty much notepad (aaah, the good old days....), but if you install all the additional plugins you can still end up with a decent environment with code completion, syntax highlighting, file (content) comparing and other such useful features. It definitely does not match up to the other choices above, but it's still pretty good nonetheless
That was my 2 cents.