That's a really tough question. Longevity counts, because you can't predict how the timing of your other pieces will work out. Adaptability to different eras or different styles of play counts too. I think I have four tiers:
Tier I -- You'd better be able to win multiple championships with this guy
James
Duncan
Jordan
Tier II -- You should be a contender most years, with a decent supporting cast
Olajuwon
Bryant
Curry
O'Neal
Tier III -- Can be a core piece of a great team, but doesn't have the "it" factor on his own
Davis
Durant
Robinson
Garnett
I'm not sure I take Garnett over Barkley, by the way. And maybe not even Robinson over Ewing.
I feel like this is one of those "we're all bored in the Suns offseason" convos and some of us are just making comments to be controversial. That said:
Absolutely no way does Steph belong in that second tier. Granted, Steph is one of the most amazing shooters I've seen. But in the same category with Kobe Bryant!?! Olajuwon!?! O'Neal?!?! No flipping way.
Are we completely forgetting that Steph can't guard anyone? Kobe Bryant was a monster on D and a juggernaut on O. Just basic common sense would say that Bryant would COMPLETELY wipe the floor with Steph one on one.
Take Steph and switch him on, say Kobe Bryant's uber crappy 2005 team. Starters: Steph, Smush Parker, Lamar Odom, Bryan Cook, and Chris Mihm. Seriously, how many games do they win? 30? Maybe. Kobe won 45 with that trash.
I wouldn't even put Steph in that 3rd tier. Given that the other 4 players next to the star are exactly average, he's not winning more games than Davis, Durant, Robinson, Barkley, Garnett, or Ewing. That guy is seriously the product of a GREAT cast and HC. And no, he's not better than Kevin Durant.