Chris asks two questions in a comment below. One difficult to answer, one impossible...!
The difficult one is 'Is his Wagner singing any good?'
Difficult, because I have only recently got into Wagner - actually not quite true, I have only recently consciously got into Wagner, but I have realised that I have been listening to bits and pieces of Wagner for twenty years. However, I know that I am in no position to make any authoritative statement on whether someone is a good Wagnerian singer. However, I know what I like, and I like hearing Plácido sing Wagner. I am currently very keen on the Wagner Love Duets with Deborah Voight, whose voice is gorgeousness itself. As Huw says, he sings Lohengrin very well - somewhere I have read that he is the best Lohengrin and the Siegmund around today. He's doing Parsifal at the Vienna State Opera next year. The Director of the Vienna State Opera was supposed to be stepping down, but isn't (I think) and chose his favourite opera and perfect cast for this occasion.
The impossible question is 'Do you have a favourite Domingo opera recording? (Whole performance, that is, not just individual aria.)'
Impossible to answer for a variety of reasons. This page lists over 180 operas, zarzuelas and oratorios on CD. I don't have a fraction of those...!
Secondly, there are so many factors that go into a 'favourite' recording of an opera. Not just one particular singer, but the ensemble, the orchestra and the conductor. And even things like the quality of the sound recording.
Another factor is the actual opera itself - for example, I'm sure he's done some fine Puccini, but I'm not really into Puccini. I don't dislike Puccini, but I can take or leave. So I could never get really enthusiastic about a Domingo Puccini because I can't get enthusiastic about Puccini. And, for example, he's supposed to be fabulous in Tchaikovsky's Pique Dame, but for some bizarre reason, I can't get into Tchaik's operas (bizarre because I love the ballet and other orchestral music...).
Another factor is what mood I'm in, or what phase I'm going through. At the moment I am completely crazy about Beethoven's Christus am Ölberge, which is an oratorio, not an opera. (As a side comment, I had not even heard of this oratorio until I read a review of this CD; looking through my diary I realised I have a ticket for next week, which I primarily booked for Mozart's Requiem, but subconsciously reckoned I could probably manage to sit through some unfamiliar Brahms and Beethoven - serendipity is a beautiful word...).
I also have recently decided that DVD was invented for opera, combining the visual advantages of video and the sound reproduction of CD, so I'm a bit hooked on Opera DVDs. My favourite of all is probably the Covent Garden Otello, which my very clever Mummy, who occasionally shows signs of actually knowing me, bought me for Christmas.
But I'm also extraordinarily fond of the film version of Carmen, which I liked so much on video I bought on DVD. (And I would highly recommend to anybody who wants to try an opera but isn't sure whether they will like it).
Through listening repeatedly to the Very Best of...CD, I am going completely nuts on Un Ballo in Maschera. I have a taped-off-the-telly video, which is of a rather strange production, with the inevitable poor quality of sound that comes from taping off the telly. But Plácido is utterly fabulous, and the final scene is one of the most OTT melodramatic death scenes I have seen. (Ad I love OTT melodramatic death scenes...!)
But you have to understand that although I absolutely adore Plácido, I think that there's a helluva lot more to opera than just one singer. For example, the fact, as I mentioned above, I'm getting seriously into Ballo, and that serendipity is a beautiful word, there's a new production of it at the Garden next season. Interestingly, the stars of this are Karita Mattila and Thomas Hampson, whom I seeing in Strauss's Arabella in a couple of weeks. So, everything is connected.
Finally, I would suggest reading what the great man himself has to say - click on The Magic of Opera.