Whether it be in terms of whether they didn’t play as well as you expected them to be, or if they were injured/unfit all the time during their career at your club or perhaps got on bad terms quickly into their career at your club (e.g. Di Maria at Man Utd).
As a Liverpool fan, I can think of names like Aquilani, Robbie Keane, Morientes, Andy Carroll, Kewell, Litmanen, Nuri Sahin etc.
But the one that has to take the cake is Naby Keita. I was very excited when it was announced he would be joining us. He was like the 2nd best midfielder in the Bundesliga that season for Leipzig in the data and for a couple of seasons had shown a lot of great technical ability. He joins us and the rest is history. It’s not to say he didn’t have good moments with us when he had a run of games in the first team. But that was presicely the issue, getting a run of games. It was a constant cycle of spending over half his time injured and when returning, not always having his best games due to slowly returning to fitness and when he did start showing his sparkle he’d go off injured again for a while. I am not just gutted for us but gutted for him too in how it could’ve and should’ve turned out.
Michael Owen was the worse signing we ever made
I was genuinely excited when we signed him - a properly world class player. And he wasn’t bad when he was fit and could be arsed, but there were so many times he didn’t want to play, clearly didn’t want to be here, and disrupted the dressing room so badly - I still think he deserves a lion share of the blame for our relegation
Patrick Kluivert was also a disappointment- though it wasn’t really his fault he was well past his best when we signed him, and at least he tried
There’s been so many underwhelming Newcastle players where I thought we’d get more than we got - Boumsong, Luque, Cabella, Thauvin, Sels, Muto - but really it’s impossible to declare that Owen was anything other than a disaster