I don't think Chuck is very overrated and neither is Couture nor Ortiz and Slice. Chuck is out of the game, Couture is one of the greatest MMA fighters of all time and now nobody would rank him in the top 10. Ortiz is known as a hasbeen and Slice is pretty much done in MMA. I think you are talking more about guys who should retire and not overrated.
If you are talking about guys who should retire, I think Couture has passed his time, which is sad because he is one of my favorites of all time. Matt Hughes is done and he has stated that he believes this as well. Coleman and both Shamrock brothers should leave as well while their brains are still in tact. Hershel Walker should end it before it's too late as well. Wanderlei Silva is another guy who I consider one my favorites of all time but he doesn't look anything as good as he use to be. Sakuraba should leave because I don't see what is holding him up.
Overrated fighters, I think of guys like Lesnar, I have no clue why he is ranked so high. He is very talented, that is undoubtedly, but he really hasn't done enough to get that spot. I consider Rashad Evans to be pretty overrated. That fight against Rampage was a fluke win and he looked pretty crappy against Thiago Silva. I can't remember the last time he looked too great against anybody, possibly against Chuck.