My list:
Duncan: Just really can't replace his defense and intangibles.
Kawhi: Has a few peers in Butler and George, but he has better versatility
LMA: Jump-shooting bigs aren't rare anymore, but those who can also play in the post are.
Manu: The play-making two-guard isn't too rare, especially with two-PG lineups being a thing. But his play-making is above that of his peers like Iggy
Green: Three-and-D players aren't rare, but they are still extremely valuable, and Danny is the best of them in terms of upside, defense and gravity. The team could get by without him, but it would take some luck.
Diaw: Point-fours aren't as rare as they used to to be, but Boris' ability to play in the post is extremely useful. Few do it better.
Bonner: Legit stretch-bigs are still rare, but obviously Bonner sucks.
Anderson: In his prime, he'll be at the top of this list. He's too inconsistent to have a ton of on-court value right now.
Parker: He's caught between roles, and right now, there are enough people who do both parts better than he does.
Boban: Size isn't replaceable, but he doesn't have a big enough of a role to fret about replacing it.
West: Jump-shooting big yawn. Love the intangibles, but the Spurs better hope he's not their best PF next season.
Mills: Three-point shooting small guard. Dime a dozen. Great value with his intangibles, but the team will probalby have to move on in the near future.
Miller: Pass-first PGs are rare, but old-as-dirt ones don't have a ton of value.
Simmons: D-leaguers gonna d-league.
Martin: Sucks.