Tim Duncan is considered by many to be the greatest PF to ever play. And he still doesn't get the respect he should.
Consider this: If you were a GM entering the draft, and you knew how their careers would play out both in terms of performance and personality, who do you draft as a rookie: Shaq, Lebron, Kobe, KG, or Duncan?
I think the vast majority of GMs would pick Duncan. Consistency. Long period of dominance. No off-court drama. No laziness. Coachable. Plays defense. Great playoff performer. Doesn't choke. For a single year, or to put up stats, you'd pick one of the others. But for a career, to build a team around? You pick Duncan.
All that said, David Robinson is probably the more valuable Spur. David put up stats like all the guys mentioned above. But unlike any of them, he stepped aside when he was still capable of putting up those numbers, for the betterment of the team.
Robinson's humbleness and team-first at ude in those few years formed the foundation of the Spurs culture today. It's what led to a team where community service and character off the court aren't just appreciated, they're expected. It's what set the standard for a star like Ginobili to be willing to come off the bench, and Tim Duncan (who I'm sure could still put up 20-10 today if given the minutes and if he wanted to) is willing to look at the big picture and hand the offensive reins over to Manu and Parker.
I think Tim Duncan would be the first to admit that Robinson helped him to become the type of teammate he is, and Duncan had the personality to be receptive to that. He also had the skill to take our team that next step forward.
Talking about the Spurs as a franchise, the Robinson-Duncan impact and relationship in terms of who the Spurs are, is impossible to ignore.