They wrote the book on endurance and can still cause a scene if and when they want to. They don't have anything to prove, having extended their careers well beyond periods where it looked over, spilled over to the big screen with critical and commercial success (in one way or another), with different eras in their careers illustrating exactly why they are legends. Madonna carved her own template (with a little inspiration from Ms. Harry), but figures like Beyonce are complete pastiche to what we have here from these three leading ladies.
Cher was very good friends with Diana for a brief moment (when Cher's career wasn't so hot and Diana's was as good as it got for her), but Diana really cannot be friends with other women. I used to think it was the Gene connection that blew it, but Simmons was Cher's hand me down and they shared another man before, designer Bob Mackie, and Cher never understood why Diana just suddenly disappeared. I do love the quote that Diana supposedly said "oh she's such a tragedy queen for sure" about Cher, but they both rolled their eyes at the other's self-inflicted predicaments. "Who cares about Cher? She'll go to the opening of a hot dog stand!" When Cher's friendship with Diana and Bette Midler (a long-running rivalry there) cooled off, Cher included drag queens impersonating them in her act. Diana's excuse for being furious was she could no longer wear a particular ensemble.
Cher and Tina have a very good showbiz friendship (it's not like Cher flew to Europe for Tina's recent wedding), but they performed a few times, most notably on Cher's highly-rated solo TV gig, and then on VH1 divas live. They refused to appear on stage with Whitney for the finale supposedly because of the drugs, but maybe also because the show just emphasized her more. Cher did pose with Whitney at the '99 Brits, so maybe that was enough. Cher recently introduced Tina's annihilation of competition winner Beyonce at some awards do or other.
As seen on Double Platinum, Diana's Every Day Is A New Day was an rn'b record that seemed to get caught in the storm as far as its two singles seemingly required to undergo dance treatments just to get noticed (the comedy Hex Hector remix is stunning). The Metro edit of Not Over You Yet would have been an understated standout on the Believe album, so it doesn't matter to me if it was blatantly turning up 5 hours late to Cher's house party (let's not forget Take Me Higher).
Diana wins the 60s as part of an ensemble, Cher wins the 70s, Tina sticks her head through the door in the 80s but Cher had a momentum that neither Diana nor Tina had ever experienced on their own, and Cher literally has the 90s and her face sewn up despite a notable slump, and Cher kind of wins the period since as she's toured extensively and successfully stays in the press without resorting to DUIs or looking like a bag lady without her make-up.
Cher's 70s work is underrated though: song-wise and vocally, Stars outshines the other two too easily to warrant any discussion quite frankly. However, Diana's singles were often actually much stronger, albeit as stand-alone tracks. Both Diana the album and Private Dancer are certainly better than Cher's 80s output. I just don't know much of Tina though.
Obviously Cher has had more success at the movies (and even she didn't reach her full potential despite bagging numerous awards, not least the Oscar), but Diana wanted to be the leading lady and wouldn't settle for anything less (least of all stoop to blaxploitation or be caught dead doing ensemble - am I wrong in thinking she was offered Witches of Eastwick? Even if not, that would have been the best film of all time for the 'chemistry' alone).
I barely know anything about Tina apart from Private Dancer, the Lulu ballad and the absolutely awesome When The Heartache Is Over (again, not much of a dance album despite pumping up the lead single to do a Cher). I really don't ever want to hear The Best ever again - it reminds me of Rangers football club and John Menzies.