Depends on what you actually want to do with it.
IMO Fusion is a bigger faff than Parallels but it's probably the most stable Windows implementation you'll get on a Mac, and is perfectly adequate for dev testing, etc.
Parallels is not terrible in terms of stability, and gives you arguably the best-integrated experience on the Mac for running Windows.
Boot Camp is supposed to give you the native Windows experience, and it does for the most part, but is notably less stable / maintainable than an actual Windows machine along with some 'is this just sloppiness or did they put this in deliberately' performance / etc quirks that you just don't get on genuine quality gear, not Apple's form-over-function interpretation of it.
To actually getting work done in Windows as opposed to Mac-grade cafe-based blogging or light dev work, I'd have to say 'none of the above'.