This question is addressed very regularly in this forum, but in summary:

As a seat to sit on CX and QF are more comfortable than BA.
As a seat/bed to lie on BA is most people's favourite since it goes horizontal and flat and the others don't.
For consistency of service CX is way ahead of the other two.
For IFE CX has an AVOD with 100CDs, 25 movies, 25 TV "features", etc. BA & QF don't.
For food CX has better Asian selections and marginally better overall.
There isn't much to choose between them on wines, although QF tend to have the best Australian selections.