As long as it fits in the criteria Usain Bolt in sprints. Winning individual titles in any given year is one thing. When the accomplishments span all time and participation that carries more weight IMO. At his best there was (even still) him and then everybody else.
100m - Has the top 3 times ever run (the first 2 outright, the 3rd is shared with 2 others. His 9.58 WR is so hilariously faster than anyone else it's still looks like a misprint.
200m - Only 3 people have run a 10.40 or under at this distance. Bolt did it 4 times. He's the only guy to even seriously scare, much less go under, 19.20.
Phelps would have to be up there as well. His medal count is just absurd. He had a ton of various WR but I think the vast majority of them have been surpassed since. Still, seriously dominant.
Rocky Marciano is kinda polarizing among a lot of people but 49-0 with 43KO's is what it is.
I waffle on team sport selections. If I had to choose it'd probably be Gretzky with Jordan and Jim Brown in the mix.