The obvious choice is the two Manchester teams, something snapped in Man City and they moved from the leader position to the 6th place and they've started easing their fans into accepting that the top 4 might not happen this season. But at least they finished half-season with a win, so there's a light in the tunnel. Man United is a whole different story. The club owners didn't learn much from the horrible last season and decided to keep Ten Hag, just to sack him after the first few games. His replacement, Ruben Amorim, is far from successful, at least so far. Man United are currently 14th.
Other teams that might be a little bit of a letdown are Tottenham and Aston Villa, but they're not as big and wealthy as the two mentioned above.
Man City for sure is the biggest dissapointment of the season. From such a good team and winning team to going to a team that will struggle to get europe. Man Utd is bad for couple of seasons so nothing new there. But Nottingham Forest is something fresh that we didnt see in a long time and to see them fighting for the title is really wierd.