1. The Zander M5 already mentioned.
2. The Levi Mahler series, especially 6 and 7
3. The Lopez-Cobos Mahler series
The Zander Mahler 9 is not well-recorded, and isn't recommended if you're looking for good recording quality.
The Mozart Requiem with Perlman and the Boston Baroque is well-recorded. The old one with Robert Shaw is one of the worst recordings (technically) I've ever heard. The recent Fidelio with Mackerras is very well recorded.
For a smaller recorded group, the Jacques Loussier recordings of classics interpreted through jazz techniques are quite good, with the best being the Bach album. The kickdrum on that recording is incredible, coming closest to what I've heard with a live jazz trio.
For my tastes, the best performance ever recorded on Telarc is the Barber album with Levi and the Atlanta Symphony Orchestra, with Sylvia McNair singing Barber's Knoxville: Summer of 1915. It's an absolutely stunning performance, and I'm always left speechless at the end. I believe it's a mid-90s recording.
Someone else mentioned Reference Recordings. If you like Mahler, have a listen to Oue's recording of Mahler's Das Lied von der Erde. It's an excellent, middle-of-the-road interpretation, with incredible recording quality. With RR, you can pretty much always count on nothing less than the best recording quality, with performances varying in quality. Keith Johnson, the recording engineer for RR, is hands-down my favorite recording engineer.