You can’t say that one person is better than all the others as it’s impossible to compare objectively by I’ll restate that Redgrave definitely up there.
He won 5 Olypmic golds – that covers 17 years. During that time he also won 9 World Championships. Rowing World Champs don’t cover events that are at the Olympics in Olympic years so that means that Redgrave won the top event of the year 14 times out of 17. In the years he didn’t win the WC, he got two silvers and a bronze. For a couple of those seasons he didn’t win, he was also competing in Bobsleigh.
He was also unbeaten from 1993 to 1996 inclusive.
And other than the first couple of WC wins (Coxed pairs – more prestigeous than it would be now but maybe not a top event), he always raced in the competitive categories. Pairs and Coxless fours. In fact, during some periods, he was so dominant that some other countries stopped putting their best people in those categories as they knew that they couldn’t win.