Well if this was an ordering poll (had to vote for gfx) they would be in this order:
Gfx/audio quality
Speed
Stability
Compatibility
...and I personally don't like user-friendliness. If it ran from a DOS prompt, doesn't matter to me. Cygwin? Fine. The more "technical" the options the more I tend to like it. Nemu is 3rd on my favorites list (because of speed and decent compatibility) but has next to no options.
PJ and 1964 are definitely top two, but I don't really know which is best. PJ does 2d stuff fine and runs SSB, but can get a little choppy (ex: Zelda64 in Hyrule Field, 16xAA). 1964 is ~1.5 - 2x as fast as PJ but has less 2d support and doesn't run SSB well.
Corn is 4th, because it's the only emu that can run Mario64 @ full speed on a P200 with no gfx problems, but has no compatibility.
Just my 2c.