I've been wondering what were the worst decisions in assigning single-byte opcodes; can you determine that from your data? I'm guessing that the decimal/ASCII adjust operations (DAA/DAS/AAA/AAS) have extremely low use considering they take up 4/256 of the opcode space.
Some of the 1-byte opcodes are specific subsets of instructions (e.g., 04 is ADD AL, Ib). Keeping in mind that many single-byte opcodes don't actually exist in 64-bit code (e.g., PUSH DS or DAS), the ones that do exist that aren't used:
ins, outs, lods, in, stc, cli, wait, pushf, popf, lahf
