This may sound like a copout answer but who you think the goat is depends on the rule set.
If we are playing with 1950's rules, small, key, strict carry/traveling rules. emphasizing Center play Youd have to say someone like Bill Russel, Kareem or Shaq is the best player under those rules.
if it's 1990's hand checking, no zone defense rules. thus emphasizing isolation scoring. MJ is the greatest ISO scorer of all time. you'd have to take MJ
if it's modern rules, modern zone defense rules, this emphaszied 3 level passing and scoring. youd have to take Lebron.