Jericho's WWF debut was definitely great. The return was predictable, but still very well done as the save.us videos recreated the millenium clock idea.
Undertaker's SummerSlam return after CM Punk defeated Jeff Hardy was pretty great. Even though everyone knew 'Taker was returning, it was pretty awesome how they slipped Hardy out of the ring and slipped 'Taker in under Punk.
Even though he hadn't been absent from the company, Sting's return to the ring against Hogan at Starcade was well done. The match certainly could have been better. But I remember, as a child of 11 years, I was in awe of the laser show at the beginning of Sting's entrance.
As far as disappointing returns and debuts, TNA could have done a much better job with Jeff Hardy's debut, although the X-Division title match against AJ was good. They could have also done a much better job with RVD's debut. Especially since it was for a match against Sting. For that matter, WWF could have done a much better job with RVD's debut as well.