for me out of the major signings (which i count 37) - not including the ones that would never play for us and are just being bought to be farmed out - I think there have been about 18 of those who have been an unqualifed success.
Dias, Laporte, Walker B.Silva, Ederson, Danilo, Stones, Sane, Jesus, Gundogan, Zinchenko, Willy C, Steffan, KDB, Sterling. Ottomendi, Lampard, Sagna
Now some would argue Mahrez - last season he was very good...but prior to that he wasnt....if he has another good season then he goes on the success list for me...likewise some would argue B.silva hasnt been ........but its all just opinions.....Some may say that Jesus has been a success BUT not in the way intended....he was brought as the successor to Kun (he wont ever be that as he aint good enough in that respect) but in his overall value to the team (and not as a goal scorer). - he has been imho a success.