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.<p>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.