Firstly, I'm just going to add in the 'Melo is not a good defender by any means. He has the tools to be, but he isn't as of right now. That's part of the big problem (among many) for the Knicks and why at this rate they won't win a title any time soon. Also, clutch-est player is very debatable; I'd say CP3 myself. (You've also got Wade, LBJ, Bryant, etc.)
Now, for me:
Overrated: Tony Parker, DeAndre Jordan, Danny Green (Sure I can think of more)
Underrated: CP3, Calderon, Iguodala, Griffin (Also I'm sure I can think of more)
I find that Parker isn't as good as everyone makes him out to be. Sure, he was a big part of the Spurs run at the title this year, but he had lots of help and didn't come up huge at any point other than once; that shot at the end of the clock in Game 1.
Jordan needs to hit free throws to elevate his game. Plain and simple; he can't be on the floor late because he's a risk.
And Danny Green... *grumble* is a scrub who got hot in The Finals and nailed a bunch of treys to make casual fans think he's a brilliant player. Had they even heard his name before that? Probably not.
As for underrated, I think CP3 is one of the most underrated superstars there is. All his work seems to be looked at, nodded at and then waved aside. That's crazy. This guy is a killer, cold blooded in the clutch. He can pick his spots better than anyone, passes better than anyone in the league right now (except maybe LBJ) and makes every single one of his teammates insanely better. His basketball IQ is through the roof. He had an MVP calibre season last year and just looked brilliant running Lob City.
Calderon is a pure point guard (there are few) and he is an excellent shooter. After watching him for years as a Raptors fan, the dude gets no love. He can run the show for you and hit open shots nearly every time. He's selective and a great passer, nearly always in the Top 10 in assists. He holds the NBA record for best free throw percentage in a season. Great pickup for Dallas.
Iggy too, gets less love because people think he can't shoot. He still struggles in that area a bit, but hey, the guy was good enough to make the Olympics, right? He is a superb defender and an amazing athletic talent who will mesh great with Curry and Thompson. Make no mistake that Iggy is a big time player; he's been an All-Star before. Also one of the league leaders in steals at his position, SF.
And Griffin. I like to defend him because, well, his ceiling is so high. Have you seen a big man with his athleticism before? Probably not in a long time. Yes, he can dunk. But he also has the tools to go to work in the post and he's starting to use them. Slowly, but surely. Doc Rivers will push Griffin next season along with Paul, and he WILL play better and learn more. The guy has unbelievable talent and an excellent motor. He brought the L.A. Clippers back to life. He is Hollywood. He helped bring over CP3. He hasn't done nothing for the franchise, people. He's still very young as well. Give him time and he will get even better than he is now; also, I find it hard to call a 20-10 guy, "overrated".