Tough question, I think Physics has to edge it because great graphics and sound cant make up for poor physics. Whereas worse graphics and sound can be forgiven if the physics feel good.
Normally I would say that graphics aren't that important, however I think the addition of full cockpit view @60fps adds a LOT of immersion. The graphics on the cars in GT5

are almost rivaling what I'm used to seeing when I play Crysis Warhead on maximum settings which is very impressive for a console. It's only the lower quality environment around the cars that give the game away as not being a photo on casual inspection.
Many people think that the sound in GT is poor in comparison to the competition and until recently so did I, however it actually improves dramatically if the volume is cranked right up. If anyone has any tips for further improving the sound I'm all ears (pun intended).
So, Physics - Graphics - Sound
Which is odd considering my favorite games actually have dated graphics (CS:S, HL2) and I'm somewhat of an Audiophile :/