I'm no doubt the only person on the planet.... but I think Dusty was over-rated and at Cilla's best, I preferred her voice over Dusty's. But I will agree re Pet Clark (brilliant, such clarity) and Dionne.
Of the four most popular British female singers in the 1960s, only Dusty and Petula had significant success in the US, the best test of an artist. Their singing ability was head and shoulders above that of Ciila and Lulu.
Cilla's big advantage was that she had a great record producer in George Martin and some good songs, although some were cover versions on American hits. Her version of the Righteous Brothers' You Lost That Lovin' Feeling was very inferior to theirs. Luckily the record buying public realised it too.
Arguably Cilla had greater success as a TV personality than as a singer.