I'm not a fan of the Grammys but if you look at the winners of the best rap album category, most of them are great albums with a good amount of classics too. And those I don't like personally are still huge commercial success so there's atleast a logic behind their wins.
1996 : Poverty's Paradise
1997 : The Score
1998 : No Way Out
1999 : Vol 2... Hard Knock Life
2000 : The Slim Shady LP
2001 : The Marshall Mathers LP
2002 : Stankonia
2003 : The Eminem Show
2004 : Speakerboxxx/The Love Below
2005 : The College Dropout
2006 : Late Registration
2007 : Release Therapy
2008 : Graduation
2009 : Tha Carter III
2010 : Relapse
2011 : Recovery
2012 : My Beautiful Dark Twisted Fantasy
2013 : Take Care
2014 : The Marshall Mather LP II
2015 : The Heist
2016 : To Pimp A Butterfly
2017 : Coloring Book
2018 : DAMN.
2019 : Invasion Of Privacy
2020 : Igor
Grammy Award for Best Rap Album - Wikipedia
The real negative thing I'll point out is that they rely way to much on Kanye or Eminem.