Definitely Victoria De Los Angeles and for one opera, Beecham's "La Boheme" with her as Mimi and Bjorling as Rodolfo, just heaven.
There are so many others too, Callas in anything, for that unique voice and the emotion in her singing, Swharzkopff for Strauss, Te Kanawa for Mozart, Cecilia Bartoli for Rossini, Sutherland, Leontyne Price. I am afraid the one mentioned I don't like is Caballe. I have one opera recorded later in her career, where her voice has the character of chalk scraping on a blackboard.