Though Rated-RKO was good, the 2008-10 run was the best. However, I zero in on his return from injury in late 2008-Wrestlemania 25 in '09. The sadistic, explosive heel gimmick while grooming Legacy as his sidekicks was great. Even with it being toned down a bit as this was the very beginning of the dreaded PG Era, it was still great.
The loss to Triple H killed all momentum IMO. Yes, he did get the title from HHH, but won it in a 6 man tag match. Then he was booked rather weakly as he soon after dropped it to Batista but got it back quickly only because if Batista's injury, got beat up often by HHH though he would escape with the belt, then eventually lost it to Cena.