vim
That said, it does depend on what kind of development you're doing. I think that in some cases, you can make life harder for yourself by straying from what most developers in your chosen language/framework are using. If I need to code up some .Net stuff, I'm going to use the MS tools. Likewise, if I'm doing heavy Java work, I'm going to likely use Eclipse (even if it's a tad bloated for my taste). If I'm on the Mac doing iPhone development, I'm going to use XCode. Sure, I could use vim for all of the above, but these IDEs are already very well optimized for their respective languages, and vim feels a bit out of place for me.