I know, there are so many recordings out there. I own many myself and I just happened to buy this one very recently as (I must confess) I used not to be a Tebaldi fan. Well, one must be objective: this Aida is first choice. Karajan's conducting is marvelous as one would expect and the singing is something legendary, like you wouldn't get anywhere nowadays. I am aware that there other Aidas (Leontyne Price is the first to come to my mind for the ravishing beauty of her voice in both of her recordings) but a few others can deliver lines like Tebaldi. Bergonzi is quite possibly the most moving voice of that generation but he also excels in more heroic roles like Radamès. Simionato is a real princess, perhaps lacking Barbieri's aggressiveness and the intensity of Gorr's but she sings with impeccable style. McNeil is probably the best Amonasro on record (hear his 'straziato' in the 3rd Act).
If you don't know this wonderful opera but you wish to have a recording of it, look no further.