Erling Haaland is already the best player to ever play in the history of Manchester City.
Scored most goals in a single Prem season and achieved countless of goalscoring records while winning the Treble as our best player. Haaland has (and will) earn multiple individual awards and will finish top 2 in Ballon d'Or race.
I don't believe longevity should matter to decide whoever is the best player to ever player in a club. N'Golo Kante spent only 1 season at Leicester City and he rightfully earned that status and as soon as he left, Leicester City were a bottom/midtable fodder.