That question is really vague. I mean, in the big picture, the best OS for programming is the one that you're targeting for your project. Shifting to a little c

That question is really vague. I mean, in the big picture, the best OS for programming is the one that you're targeting for your project. Shifting to a little closer of a view, the best OS is the one that makes whatever sort of programming you're doing easier. But "best" can mean a lot of things. Personally, my favorite OS in terms of how enjoyable it was to program for (but absolutely not in terms of performance) was Newton OS.