Benchmarking LLVM/Clang 3.2, GCC 4.8, DragonEgg Compilers
John The Ripper favors GCC over LLVM's Clang due to LLVM/Clang not yet having mainline support for OpenMP, although the support is being worked on by developers.
GraphicsMagick is another program that heavily utilizes OpenMP so the best performance is with GCC.