Russell Westbrook? He went from being like a 3-star recruit to what he is today. Karl Malone too; his workouts as he got into his mid and late thirties are legendary.
This is NOT to discredit the likes of Jordan, Kobe, Tiger, everyone else on that list. But personally I'm leaning with Jerry Rice.
Jerry was running just as fast in the fourth quarter of an NFC championship game as he was on the first snap of the regular season. For not having outstanding speed (though he was fast enough) or being physically imposing, he had stamina, core strength, and agility in spades. He became impossible to predict going into a route; in his NFL Top 100 video they talk about how defenders had no idea what he was going to do because all of his routes looked the same for the first five yards. Jerry Rice was a machine.
Masters of Habit: The Deliberate Practice and Training of Jerry Rice
If you split Jerry Rice's career in half into ages 23-31 and ages 32-42, here's what it would look like:
Jerry Rice 23-31:
All time TDs: 118 (5th all-time)
All time receptions: 708 (45th all-time)
All time receiving yards: 11,776 (29th all-time)
Yards/game: 84.1 (5th all-time)
Rec/game: 5.1 (in line with Brandin Cooks, Amari Cooper, A.J. Green from this past season)
Eight Pro Bowls, seven First Team All-Pros
Jerry Rice 32-42:
TDs: 79 (27th all-time)
Receptions: 841 (27th all-time)
Receiving yards: 11,119 (34th all-time)
Yards/game: 68.2 (30th all-time)
Rec/game: 5.2 (in line with Brandin Cooks, Amari Cooper, A.J. Green from this past season)
Five Pro Bowls, three First Team All-Pros
Accolades included, both Jerry Rices would be top 25 wide receivers of all time and Hall of Famers

do your wind sprints