To be able to use an editor for everything, it needs to be the right sort of editor. It needs to be flexible, extensible, and light (i.e., fast to open). I'll take those points one at a time:
Flexible
"Everything" covers a broad gamut and so the editor has to be flexible enough to be able to work well across that broad gamut. Things like search, syntax hilighting, splitting, keyboard-shortcuts, multiple-tabs or open files are features I'd put in the flexible category. There's also that intangible of how it feels in various languages and scenarios. Does it, for instance, feel as good to use when coding C++ as HTML as Ruby as Plain text? It's got to be flexible.
Extensible
No editor can come bundled with everything you need, so an easy, flexible extension system is required. The system should be powerful enough so that you can build a feature if it's missing, or even a whole new suite of functionality. It should also be easy enough so that you don't have to think too hard in order to create quick and dirty extensions and macros. It's got to be low friction, yet powerful.
Light
One of the most annoying things is when you have a powerful, flexible editor that does all these neat things and you love to use it all the time, and yet it takes about a minute to load. You develop a kind of friction against it when you just want to quickly pop open a text file. In order to be able to use your one editor and use it well, it must be light enough so that you don't even think about popping it open to quickly view a log file or readme. It's got to be a frictionless experience. It's got to afford quickly opening, closing and moving around the screen (if you're on a GUI).
With these points in mind, what would I use on each platform?
TextMate (Mac OS X)
A perfect candidate for the one, true editor. Has a remarkably powerful extension system that takes advantage of underlying OS shell capabilities and scripting languages. It perfectly embodies the flexible, extensible, light mantra.
E Text Editor (Windows)
Essentially TextMate for Windows. Innovative use of Cygwin means it can take advantage of the power of the *nix shell tools and scripting languages. It even contains some things that TextMate doesn't have, such as Snippet Pipes.
Vim or Emacs (*nix)
Old-school, sturdy, reliable, time-tested. You see an experienced touch-typing hacker working one of these editors and it's like observing a 10th dan kung-fu master. These don't have the slick extension system of the first two, but who cares? The extension system is the underlying OS. The possible downside to these editors is the learning curve but, really, it's well worth the effort.
So yes, I do use one editor for almost everything. I do usually switch to TopStyle if I'm doing CSS, though. I'm mostly on Windows at the moment, so I mostly use E Text Editor. But if I'm on a Mac I'll use TextMate. If I'm on Linux I'll use Vim. The author of E Text Edtior has said he'd like to do a Linux version eventually, which would be awesome. Can't wait for that.