I think anytime someone uses the phrase "greatest QB", there needs to be a caveat. Because there's no single QB that ever played that was the best at everything.
Most talented? Marino, Elway, Vick, Baugh, etc.
Most accomplished? Brady, Montana, Bradshaw, Graham, etc.
Best at running an offense? Manning, Unitas, Montana, etc.
Had the best coaches? Brady, Montana/Young, Starr, etc.
The debate goes round and round because a QB can't just hang their hat on one thing (like SBs, which is a TEAM accomplishment) and say that's why they're the best.
Andrew is arguably the most talented QB the Colts have ever had. But he's not as accomplished as Manning or Unitas (yet). He has time to prove he can run an offense like they did. And maybe Reich will be the kind of coach for Luck that other great QBs have benefited from in the past (and currently).
Bottom line: Andrew Luck is only halfway done, and the second half of his career will decide his place in the pantheon of NFL QBs.