I would go with QT, it's easy to make video for and everyone has the QT plug in. WMP is shit on Mac's and other systems cuz you have to boot in to the whole overbloated application, and as I said before, despite problems QT may have on other platforms WMP sux generaly.
RealMedia is cool and portable too, but the problem is that .rm's generaly suck ass and their player is probably more unmanagable than WMP's. Your choice, id go with QT. You don't have to make 'em .mov files for them to play thrugh QT you can have .mpg's and stuff, I dunno, I like it the best.