It's difficult to recommend just one Luisa Miller recording. All have their strengths. This is defenitely a safe all-around choice.
Caballe, still in her prime, shows what an intelligent singer she is. In the beginning of the opera she is sweet, just as she should, but she changes her voice as the action proceeds. She does not always sound very youthful, however. I found her surprisingly strong and daring in her big scene. Usually she avoids singing high powerful notes but not here. The first Luisa I heard was Millo, who is good but one immediately notices the difference in the quality of the voice when Montserrat sings. Voices like Caballe's are rare.
Pavarotti is also still in his prime and makes an excellent Rodolfo. I've never been a Milnes fan because I don't like his voice but it's hard to deny that he captures the drama well. The rest of the cast is also good though less famous. I'm not very familiar to P. Maag but his conducting is fine, allowing the singers to perform.
From the other Luisa Miller recordings one should also consider the early RCA set with the convincing youthful voice of Moffo, the sublime Bergonzi and a great supportive cast. Ricciarelli as Luisa Miller is a must but she recorded the role in studio a bit late; try to get one of her live performances (with Carreras or Pavarotti as Rodolfo). In any case, the Decca set is easily recommended as a great Luisa Miller recording. Note that this version offers libretto accessible via your PC so you'll have to print it out yourself.