well, i've used mupen, pj64, 1964 and nemu64 and i would have to rank them like this
Project64>1964>Mupen64>Nemu64
Project64 is just more stable, other than that 1964 matches it in every way, mupen is essentially a clone of pj64 without support for gs codes. nemu is more of a developer's emulator if you ask me... it has tons of debug features and its graphics plugin exports 3d geometry.