Level of dominance:
This is a category that is impossible to conclusively judge – how does one assess one’s level of dominance? Is it the winning percentage? Is it the medal haul? Or number of finals reached? Is it the margin of victory?
If winning percentage is the way to go, Bolt is victorious. The Jamaican has a 90% winning percentage in Olympic Finals (9/9 since Beijing, but he failed to make the final at Athens as he finished 40th overall); whereas Phelps has a percentage of 82%, winning 23 out of 30 finals. Granted, Phelps has a much larger scope for failure given he has completed in three times the number of finals as Bolt, but if winning percentage is to be believed, Bolt wins.
However, that said, Phelps has a 100% record of making it to the finals (in other words he has never failed to make an Olympic Final, even as a lanky 15-year old in Sydney competing in the 200m butterfly). In comparison, at 17 years old, Bolt finished fifth in his heat in the 200m, and subsequently failed to make the final. Despite being the only blotch on any otherwise perfect career, in this department he loses to the American.
Margin of victory would probably favour Bolt, who could afford to eat chicken nuggets, run with his shoe-laces open, stop running for the last 20 metres, and smile at the camera ALL while winning gold in a world record time, whilst Phelps was lunging frantically at the wall and snatching victory away from Milorad Cavic by 1/100th of a second. However, is that really relevant? Surely the aim is to firstly win the race, and then hopefully break some record and if you happen to do that by annihilating the opposition, or by the smallest of margins is irrelevant, surely?
The extent of dominance would be Phelps though, and nothing Bolt did would ever change that. Phelps has mastered all 8 races, involving a mastery of all four strokes, two of them at three different distances (50m, 100m and 200m), and in doing so, his dominance spreads further than that of Bolt.
Verdict: Tie, but if I was pushed, I would have to say Phelps.