Highlighted comments from commenters below (Im sorry Montse)
+Lellobeetle Callas herself knew very well that many people didn't like the sound of her voice, and she herself was among those people. She said that the first time she heard her own voice on a recording she almost cried her eyes out and wanted to stop singing. John Ardoin, who was the most informed writer on Callas as well as a personal friend said that once Callas asked him, "When you first heard my voice, did you like it?" He said, "No." She said, "I didn't think so."
For me, there are voices that are beautiful, and there are voices that are better than beautiful. Similarly, there are actresses that are beautiful, and there are actresses that are better than beautiful (think Streep, Magnani, etc.) These singer's voices might at first be disquieting, but they are able to express so many colors and emotions besides beauty. Carlo Maria Giulini likened Callas's voice to a string instrument: "Her voice was a very special instrument. Something happens sometimes with string instruments—violin, viola, cello—where the first moment you listen to the sound of this instrument, the first feeling is a bit strange sometimes. But after just a few minutes, when you get used to, when you become friends with this kind of sound, then the sound becomes a magical quality. This was Callas."
And although not the prettiest sound in the world, going by contemporary descriptions, it seems that it was eerily similar to the voice of Giuditta Pasta, for whom Donizetti wrote Bolena and Bellini wrote Norma and Sonnambula. Henry Fothergil Chorley wrote about Pasta: "There was a portion of the scale which differed from the rest in quality and remained to the last 'under a veil.' ...out of these uncouth materials she had to compose her instrument and then to give it flexibility. Her studies to acquire execution must have been tremendous; but the volubility and brilliancy, when acquired, gained a character of their own... There were a breadth, an expressiveness in her roulades, an evenness and solidity in her shake, which imparted to every passage a significance totally beyond the reach of lighter and more spontaneous singers... The best of her audience were held in thrall, without being able to analyze what made up the spell, what produced the effect—as soon as she opened her lips".
If one didn't know whom he had written this about, one would naturally assume he was describing Callas.
Since you are a musician, I hope you give Callas more chances. Just listen to her sing the Mad Scene from Bolena, and just listen to what she does with the music and the words. She sings Donizetti and Bellini the way Guiomar Novaes played Chopin, with absolute mastery of phrase, a incredible yet understated virtuosity, and an architectural sense of line and of the rhythm within the rhythm of the music. Don't just listen for beauty of voice (though there is plenty there), but listen for the beauty of the singing and of the musicianship. You might be transported to a level you didn't think was possible.
+Lellobeetle Listen to her D'amor al dolce impero from 1952 Armida in Florence. Or to her D'amor sul ali rose from Trovatore a La Scala in 1953. Or listen to her Casta Diva from the first EMI set in 1954. Or to her Pace Pace Mio Dio. These are all before her 80 pound weight loss or shortly thereafter.
As for comparing Callas and Caballe, Montsy would be the first one to tell you not to. She idolized Callas and often sought Callas's help in some roles. She said, "That I am compared to Callas is something I never dreamed of. It's not right. I am much smaller than Callas."
A friend of mine once met Caballe at a party and when conversation turned to Callas, Caballe said two interesting things: One was that when Callas looked at a score, she immediately saw all the shades and nuances and all the unwritten things, and she couldn't understand how they weren't so obvious to anyone else. Second, she said that whereas it took Montsy a week or so of practicing to get a difficult phrase into her voice, Maria could look at it, hum it to herself, then sing it perfectly in full voice.
You can hear an evidence of it in her Armida of 1952, a fiendish role which she learned in a week and performed, leaving a definitive interpretation behind.
As for pianists, if you haven't heard Novaes, search her out. I find her Chopin incredibly moving, and her version of his first Piano Concerto is my favorite. She's not schmaltzy, and she's technically flawless. Most of all, she truly understands the music and plays it as if she were talking to you. Just as Callas sang some of the more difficult passages under the breath as if they weren't even there, Novaes underplays the virtuosity, but brings out the music and the feeling.