generally, there are no good M9 bayonets, and never were. The M7 is great, the OKC are good. The M9 was a AKM knockoff, but crappier. Get an M7, S3 if you want a decent bayonet. The m9 is a swiss army knife that never worked.
I could be wrong, I never used an M9. But that seems to be the consensus. My experience with the M7 is good, the S3 is decent. Lots of people complain about the M9, but I never have seen one. The Rattail seems to bother people. The M7 was the last of the true bayonets, while the S3 was a fighting knife that was also a bayonet, the M9 was a utility knife that was neither bayonet or combat knife. I'm sure it had its value, but it wasn't recognized as a weapon, and was known to break when worked hard.