BC2 will be the winner....the sound effect of ur ears gettin numb after a blast or explosion is awesome...
All very good, but BFBC2 brought a whole new set of ideas and possibilities to sound in games. I'm a professional game audio designer and that's the one game that got me and lots of others thinking "what? how they did that?". Amazing use of real time signal processing you won't find anywhere else.
Halo: Reach gets my vote! How the f*ck is it not in this list?! It's one of the things Bungie do best is sound! Just listen to it when you play that game!
BFBC2 for 100% RDR comes second. However, as all lists, there are some games who should be added here too.
vanquish's sound appears to be pretty good, and limbo's and RDR's is also quite excellent. But BC2 should really win in this category, as it sounds like you're in actual war. it's really draws you into the game like no other.