They're not 0 out of 10 - they would have been, if they'd taken money out of the club. At this time, they're probably a 5 out of 10 - 6, if you're being exceptionally generous, imo. Just average and somewhat pointless in most ways.
The players we actually signed? Depends on the window, really.
Summer 2010, January 2011 - 6.5 on 10. Some great hits like VDV and Sandro, a dependable free transfer in Gallas. However, critical investments in a powerful, line-leading striker were not made, and the Crouch/Defoe partnership dried up in effectiveness that year. Loss at end of window.
Summer 2011, January 2012 - 2.5 on 10. A couple of good deals in the summer in Adebayor and Parker, and then some utterly useless freebies in Saha and Nelsen when a little bit of investment in quality would have pushed us over the line in January 2012. Major profit at end of window.
Summer 2012 - January 2013. 5 out of 10 - a mixed bag. Some long-term successes in Lloris and Dembele, but missed out on the players AVB desperately wanted because Levy cheaped out (Moutinho, Hulk), and their replacements (Sigurdsson, Dempsey) were inadequate and quickly moved on. Minor profit at end of window.
Summer 2013 - January 2014. 3.5 out of 10. The only one of the Fab Seven AVB actually wanted as a first choice, as I recall, was Paulinho. Everyone else was a bit of a crap substitute for a player he actually wanted (Villa, Benteke, Hulk again, Witsel, etc.). And they failed accordingly - Eriksen and (perhaps) Lamela aside. Minor profit at end of window.
Summer 2014 - January 2015 - 1 out of 10. Sort of an early warning sign for Poch about how Levy would operate. He came in wanting Schneiderlin (who put in a transfer request and kicked up a fuss to move to us), Jay Rodriguez and Hector Moreno. Got absolutely none of them, ended up with the useless bargain bin Stambouli and Fazio, who were both unceremoniously booted out after a year. Minor profit at end of window.
Summer 2015 - January 2016. 7.5 out of 10. Probably Levy's best transfer window, and his ideal - ended up making a profit on it at the end. bought in Son, Toby, Trippier and Wimmer - the former three would prove crucial cogs in Poch's best sides in the coming years. Lost out on a number of other first choices (Berahino chief among them, but whatever). Minor profit at end of window.
Summer 2016 - January 2017 - 4 out of 10. Pretty sub-par windows. Bought Wanyama, which was great - lost out on Mane and Wijnaldum, and ended up with a panic buy in Sissoko, which was not. Cheaped out on our striker targets, ended up with Janssen. Loss at end of window.
Summer 2017 - January 2018. 3.5 out of 10 - Beginning of the end. A great buy in Sanchez, overshadowed by the loss of Walker and replacement with Aurier. Good pickup of Llorente, marred by the bargain bin pickup of N'Koudou instead of a proper winger. Got Lucas to compensate in January, but the damage at the start of the season had been done. Minor profit at end of window.
Summer 2018 - January 2019 - 0 out of 10 - if it made sense to give this window a negative, I would. A cowardly dereliction of duty, self-sabotage of the highest order by our penny-pinching, risk-averse leader. Signed no one, and we are still paying for it now. Minor profit at end of window - obviously.
Summer 2019 - January 2020. 5 out of 10. Tried to make up for years of underinvestment, bought a great player in Lo Celso, two unknowns in Bergwijn and Sessegnon, and a struggling, increasingly at-risk player in Ndombele. Major loss at end of window - probably the first one in ten years.