He has all the tools, but declaring him on par with the greatest defenders in NBA history is an insult to them and their careers. Smart may surpass Gary Payton one day as a defender, but he's not there yet.
Smart is quickly becoming the most intimidating defender in the NBA. At some point, the legend of Smart will get even bigger, and then Smart will have a psychological advantage. As players stop viewing him as a young spastic wanna-be, and start fearing matching up with him, they will play tighter, force more bad shots, and become more erratic in their decision-making.
What Smart does more than any other defender is win the mental game (and that is what great players do). He can put you into foul trouble with two quick offensive fouls. He can steal it from you at any time. If offensive players stop playing relaxed and start playing afraid, or tight, or overly aggressive, then Smart deserves an All-Defensive first team.
If he does that over a 5-8 year period, then he is an all-time great.