My opinion:
SQ: for comfort and for *A miles, I think it's worth the slight premium.
EK: comfort comparable to SQ, but I hate the wee hour stopover. Miles are restricted to EK.
BA: not the best in comfort compared to the above 2 carriers. If you want a direct and cheap flight, then this is for you. And also if you're a member of One World.
Edit: to add on, if you're flying on BA's B744, they're really noisy compared to SQ's A380 and EK's A380/B77W. I find it harder to sleep comfortably on BA with the noise level.