Given perfect tuning and all other things being equal, the head with the smallest amount of surface area will in theory be the most stable as it should have the least friction/interference against the wind and air as it's shot. By this logic it would seem that 2-blades will in general be most stable.
That said, the aerodynamics of how things fly can vary greatly with different angles, thicknesses of blades, length vs width, etc so the only real way to test would be to analyze heads side by side. I wouldn't be surprised to see some really well designed heads with 3-4 blades surpass some crummy 2 blade designs.
Also, just as a matter of reality with our equipment so very limited on speed and distance that we can hunt at, I would wager that the flight stability variances between broad head designs would make little to no measurable difference in accuracy, lethality, or penetration for any of us that aren't Olympic athletes, Byron Furguson, or the like. I choose to shoot heads that maintain my accuracy, are easy to sharpen, are durable, and that gets the job done. For me that's Grizzly single bevels. I've shot others in the past but my needs and preferences have changed a bit over the years as I've altered my arrow builds and gained more experience and knowledge. I can't say that I've ever shot a "bad" broadhead of the bunch that I've owned, I've just had some that tuned and worked better than others.