It has to be Tiger. Golf is so much more accessible to the masses these days you have to wonder if there was anyone out there during Jack's era that just didn't have the opportunity to play.
Also, when Tiger is hitting on all cylinders there's not a player in history that could match him mentally and physically.
Let's be honest, Tiger is 31.. Has 13 majors and 61 tour wins. Unless he gets hit by a bus or is diagnosed with a career threatening illness (GOD FORBID!!) he's going to win at least one major every two years. Which means, we'll only have to ask this question for another 10 years
Don't get me wrong, Jack is a legendary golfer, but I have to give the nod to Tiger for greatest of all time.