Obviously all subjective to your age
As a 50 year old bloke, I have a little trouble with people who people nominate Maradona for football, as you never got to see him, bar a WC every 4 years, and the very odd occasion Football Italia showed Napoli, on a Sunday. There simply wasn't enough exposure, at that time.
So, as 50 year old
Football. Phil Jon..... sorry, Messi
Golf. Woods #
Boxing. Tyson##
RL Hanley or Langer... can't make my mind up
Cricket. Warne. You just breathed a MASSIVE sigh of relief, when he wasn't bowling
Then there's the sports I couldn't give two fucks about, but still have seen.
F1 Senna
Tennis. Federer
Snooker. Hendry
Darts. Taylor (spits)
Athletics. Michael Johnson (he made it look easy)
# at the risk of contradicting myself, because of age, I'd go for Jack (watching old, readily available repeats). He was playing week on week against legends, Watson, Palmer, Player, Seve,... Woods just used to beat up Mickelson, Els, Singh, etc. There's a world of difference
##again, because of age contradiction, I'd go for Ali. Again, in his time, there were greats. Tyson just used to hammer fat Yanks like Jesse Ferguson, etc. The heavyweight division was poor, when Tyson was in his pomp