I'd say January 2023 was our worst window.
Things were still salvageable, but wildly, instead of going for a goalkeeper, we focused on trying to spend over £20m on Harrison because Rodgers thought we needed another creative player! Yep, Ward had a decent run before the international break but was woeful again after the league resumed.
Not only that, all 3 signings we made - Tete, Kristiansen and Souttar - were failures. The fact we had such high wages, and were expecting a European spot started the spiral. Bournemouth had similar challenges and they signed Zabarnyi, Dango, Vina and Semenyo, while Forest went all in to get Navas from Real Madrid - both sides finished above us.
I'd say Summer 2024 was our 2nd worst, we're still paying the price now but at least we got Bilal who was decent signing, albeit not particularly loyal!
And then the Summer 2016, when we spunked the opportunity to build on our PL winning season, and wouldn't get to competing at the top for another 3 years.
Summer 2022 was pretty bad too - all the signings failed - which meant we failed to get European football, and screwed us when it came to PSR, leading to a lack of activity in Summer 2023, and an alienated Rodgers. Actually, think that's worse than Summer 2016 as at least we recovered from it within a few years.
So overall:
1. January 2023
2. Summer 2024
3. Summer 2022
4. Summer 2016