With close to a thousand SACD/Hirez recordings, I feel like I can just now start to maybe broach this subject. But man, oh man, it's such a HUGE topic, I barely feel up to the task! Maybe I'll just start it and then others can contribute too. OK, lets begin.....
As a big classical guy, I have a ton of these recordings in standard rez, and I've had the chance to compare the standard rez to the hirez directly on a track by track basis, so I can get an idea of the differences going on. For the most part, things that sound great on redbook, also sound great (and maybe even a bit better) on hirez. On the other hand, I think older recordings is where there are some spectacular gains in sound quality. Many of these older recordings sound amazing in remastered hirez. However, not all of them do.
Of the new recordings that sound great, some are no-brainers, like any of the Hilary Hahn recording, or any of the Tilson Thomas Mahler recordings, or any of the Paavo Jarvi Beethoven recordings. Or the recent Tokyo String Quartet's Beethoven, or any of the clarinet music performed by Frost (that guy is amazing). Same for Ivan Fischer - any of his recordings are outstanding and deserve purchasing.
But what about older recordings? I can say this - they ALL benefit from hirez remastering. But some are so poor to begin with, that no remastering will ever salvage them. However, there are some that have really surprised me and I would heartily recommend checking out. Almost any recording by George Szell, Heifetz, Bernstein, Solti, Bruno Walter, Karl Bohm, Kempe, Pollini, Argerich, or Karajan would qualify. Each of these, IMO, were particularly poorly served on redbook, as they were each sort of assigned a "sound", and all of their recordings were mastered on redbook to match that preconceived sound. On hirez a lot of that falls away. For example, Solti is always thought of as hard edged and driven. But on hirez, his humanity comes through. With Szell, it was always as a perfect taskmaster. But on hirez he has real feeling, across the spectrum. Karajan is thought of as an egocentric, but on hirez it is clear that he has a real feel for the sheer beauty of the music he is conducting. And so on and so forth. Oh, and I have to give a shout out to both Solti and Furtwangler (RAIA) recordings of Wagner's Ring on Hirez. Both are freaking awesome, in their very different ways. Note, however, that most Furtwangler recordings suck, even in Hirez - the rome Ring recordings are an exception. But what an exception!