Parker is an amazing point guard that can put up 20+ points and 8+ assists easily on any given night. Speedy, good shooter, good defense, good basketball IQ..pretty much everything you can ask your PG to be.
Duncan, on the other hand, holds even more court presence than Parker. Even while old, he can still out-rebound the young guys and easily get 10 boards and 15 points a night.
Melo, is a scoring champion, who excels at, scoring. Other than that, he doesn't assist, he's a mediocore rebounder for his size, and subpar defender.
I'd rather have players that take efficient shots, plays good defense, and excels at passing / rebounding rather than a player that gets 30 points a night on 25 shots and doesn't excel in anything else.
I still think Melo is the most important Knick on the team and easily the best player here. But to compare him to Duncan / Parker and saying he's better is a bit of a stretch.