I like Luka best of these three. Winning also matters though. Giannis already has a quality team around him... while we can only guess if Dallas will continue to improve the roster around Luka... and if Jokic will get the help he needs in Denver.
Btw, I think you're missing Shaq from the GOAT candidates. The three greats from that generation were each great in different ways (Shaq: physical dominance, Kobe: pure talent, Duncan: all-around consistency). None of these three entered the GOAT debate but Shaq came closest. Ultimately, he screwed himself by becoming lazy and changing teams. It's the only thing that brought him down to Duncan's and Kobe's level, when he should have retired at least a tier above with his level of physical dominance. Who knows, Shaq could have ended up with 6-7 rings in a row as the 1-1A if he'd stayed lighter and been less toxic.
Also, if you never saw Shaq in person... words just can't describe it. He looked big enough to eat Duncan for breakfast, Garnett for lunch and Kobe for dinner. TBH, if the four of them were stuck on a boat like The Life of Pi, that's probably what would have happened.
