1) 2014 Live German recording of Starlight Express - Love the updated orchestrations and I think this iteration of the production captures the best of all the variations comprehensively. And it's the only recording (I believe) to feature the new love duet, "Fur Immer" (I Do). Special shoutout to the original German recording which also has its' charms (some of the very disco-sounding numbers are delightful).
2) German recording of Sunset Blvd. - Helen Schneider (although the youngest Norma on record, I believe) gives a searing portrayal of Norma that is one of my faves. Her pop single version of The Perfect Year is also worth tracking down, I think it's probably my favorite non-show recording of that song.
3) Canadian Sunset Blvd. - Diahann Carroll provides a stunning alternate take on Norma. I find her to be the strongest Norma, which is an interesting choice that seems to work well. I also almost always prefer the art direction for the Canadian editions of shows. The poster for Sunset with the addition of the purple and other Hollywood elements is stunning. That is not represented in the album booklet, but it does contain other key art that is different from the usual Sunset fare and very lively and engaging. I also find the orchestration here to be the only recording that most closely resembles the "live theatre" experience.
4) Canadian Joseph - Although I much prefer Linzi Hateley to Janet Metz, Donny Osmond really does give an excellent performance along with his supporting cast on this recording. Again, my love of the art direction for Canadian editions of these shows contributes to my love for this one. The only problem with the Joseph recordings is they all use the same backing tracks, so we're not exposed to all the nuances the various casts employed in their stage versions.
5) Pearl Bailey's Hello Dolly - This probably isn't "unknown," but it's certainly harder to find. For obvious reasons, this recording is so worth keeping in regular rotation.
6) Vanessa Williams' Kiss of the Spider Woman - Just when you thought the original cast couldn't get any better, this replacement cast (with Howard McGillan and Brian Stokes Mitchell) comes in. This recording is great because it really feels like you're live in the theatre listening. The orchestrations feel much more organic and less-polished, which I prefer. It does contain more music than was on the original cast recording as well (I believe there's another Over The Wall reprise and the whole denouement where Molina completes his mission).