In my opinion YMD is the most useful because it is the most visually distinct from the other two. A date like January 2nd 2012, when arranged in either MDY or DMY format, is difficult to distinguish from February 1st. YMD on the other hand is obvious, independent of what the reader is used to.
MDY is syntactically correct English: January 30th, 2012. Or in older form (where it would have come from): "Dated: January the 30th, in the year of our Lord 2012." Even the US Constitution is dated this way. I strongly prefer this way because it makes the most sense in the English language.
For programmatic sorting YMD makes sense because is automatically sorts by Y, M, D, if compared as a string. (EG: on a filesystem.)
