If you only have a stereo system, you should go for one of the stereo mixes.

I love the mono mix myself, but as far as pure audio quality goes, the stereo mixes are superior. But the mono mix has so many unique differences that it's at least worth a listen all the way through once.

Personally, if you want the best overall audio quality, I'd go with the 1977 stereo mix. As CatBus said, the 1985 mix (which is the one I grew up with) has limited dynamic range, though the 1977 stereo mix has a narrower stereo field, so frankly it's kind of a toss-up between 1977 stereo and 1985 stereo. I'd still give the edge to 1977 stereo.