I agree with you that Colin/Christie, Rob/Amber, Dustin/Kandice and Eric/Jeremy are among the finest teams that have ever competed in a U.S. Amazing Race.
I have an additional suggestion, which is Zach combined with any strong female racer. If we limit it to the U.S. then I would choose Rebecca of AR6. However, if you open it up also to Amazing Race Asia racers, then Pailin would be an excellent partner for Zach.
I also think that Nat and Kat have earned the right to go up against the very best teams of the Amazing Race.