[Answered in March 2017]
Visual Studio, 2015 currently to be precise.
I have been using Visual Studio for about 20 years now, and I can say that it is undoubtedly the best IDE out there. In fact, after using VS, it gets difficult to adapt to any other IDE.
Its code completion and refactoring features are top-notch, but what seals the deal for me is IntelliSense. In fact, I have become quite addicted to it and — owing to this one feature — I cannot imagine moving to any other IDE.
And then, of course, there’s C#, a language that I have seen evolve from a wannabe clone of Java into an avant-garde testbed for exciting new ideas. I use it not just for its object-orientedness, but also for the fact that it lends itself so well to functional programming.
All in all, C# with Visual Studio is a power-packed and winning combo for me.