I agree that Holmes was an excellent technician gnu. He was a big man with a percussive jab, who was not easy to hit.
The best I have watched over a longer timespan was probably Lennox Lewis. He beat every man who ever entered a ring with him. Both his defeats were convincingly avenged. He was a tremendous finisher, but he also had the skills to outbox men like Tua and Mavrovic over the distance if necessary. I will be surprised if I live to see a better fighter. Ali's skills were probably the most spectacular, but he spoiled his record by boxing when he was way past his prime.