HAVOK. It is done entirely in software with great results and no vendor lock-in.
PhysX may be better overall, but so far every single tech demo or actual game implementation is filled with pointless effects that suck GPU performance for shiny lights and other cheap tricks such as waving flags, volumetric smoke, etc.
CHEAP tricks? Tricks maybe, but cheap? No.
Go play something like Borderlands 2 or Arkham City with Physx on. Sure the effects aren't needed and sure they don't affect game play but it ADDS to the realism and over all beauty and atmosphere of the game.