With a heavy heart, I must say Visual Studio. Despite being a closed source, bloated behemoth it is by far the most easy to use. Intellisense is crazy useful, and the plugins available (FxCop, CAT.NET, IBM AppScan) are extremely well put together.
Outside of that, I'm a proponent of Notepad++ and Textmate. I never really got into the whole vim/emacs thing, thought I often wish I had a working knowledge for when that Solaris box hates me at work.
