This could be for personal reasons like an emotional connection, or the grand scheme of the decade as a whole for music/culture
I think To Pimp a Butterfly came at the perfect time for the music industry and the world due to social and political issues, for example
While Blonde is probably the album of the decade that i have the closest emotional connection to,
or for instance whatever you think or know you listened to most
My beautiful Dark Twisted Fantasy is the greatest rap album ever and hasn’t been topped since it came out
3 #1 singles and diamond in the US alone. sounds like a decade defining album to me.