For me its a toss up between s7 and the reunion.
The first half of season 7 had potential because it still followed up storylines from season 6B, but s7B was appalling and a massive blot on the later years, which lead into season 8.
However the reunion is probably more problematic for me since the reunion came on the back of the renewed and refreshed season 9, the strongest season in years, and it was like they reset everyone's personalities back to pre season 6B.
Sammy Jo and Steven fighting again when by the time Steven left at the end of s8 the pair were practically a couple and were getting along much better than they had done in years by that point.
Sammy Jo who was one of the few, if not only character, that had actual character development and had grown and matured into a lovely young lady befitting of the Carrington name had reverts back to her bratty early 80s teenaged persona.
Fallon and Steven barely interacting! Seriously not one scene with them catching up! The Fallon we know would have been annoyed at Steven for not speaking to her or the rest of the family for all those years (I think its implied that Steven has actively stayed away from the Carrington's since he left Denver back in 1988) this would have been the perfect opportunity for Emma Samms and Al Corley to share a scene together too, bothe Dynasty actors from vastly different eras of the show.
Revisiting the long resolved Jeff/Fallon/Miles triangle, I mean seriously! This storyline wasn't even on DYNASTY it was a COLBY'S storyline which had been resolved half a decade earlier, the only Colby's I wanted to see was Sable since she was a regular in season 9 after all!
Steven being irrationally angry at Blake, Steven knew Blake had long accepted his sexuality and Steven having a boyfriend.
No mention of Claudia, Amanda, Dominique, the rest of the Colby clan, a throwaway mention of Dex which was vauge at best.
Things I did like was that Steven was finally in a happy gay relationship.
The Adam and Kirby storyline revisited.
Kirby finally getting her cat fight and showing that she was stronger than when we last saw her.