I have terrible taste so take my opinions with a grain of salt.
Mindwebs: This was a show produced by a guy named Michael Hanson for a radio station in Wisconsin back in the late 70s and early 80s. He simply reads sci-fi stories with some additional background music. He has a good voice, and the stories are a nice mix of classics and more obscure fare. Hanson recently returned to record a few more episodes before his recent death. These are slowly being released on the Internet Archive.
A few of my favorites:
049 Light Of Other Days
177 Rust
109 The Man Who Lost The Sea (The Escape Pod version is brilliant)
094 But As A Soldier For His Country
006 Kaleidoscope (I'd recommend listening to the dramatization by Suspense from (07.12.55) first)
037 The Star
069 The Last Question
115 The Fog Horn
034 My Object All Sublime
178 Warning Signs
Suspense: A classic and very long running Old Time Radio program that usually specialized in crime stories but occasionally branched out into other genres. "Kaleidoscope" (07.12.55) is a brilliant ensemble piece. They also did a nice adaptation of "Donovan's Brain" (05.18.44).
The Truth: This is a modern podcast. The episode "The Dark End of the Mall" is one that I really liked.
2000x:
These were produced by NPR and were hosted by Harlan Ellison. I should have loved this series, but I really didn't care for it. It seemed that almost every episode was done in a tongue-in-cheek manner and it rubbed me the wrong way.
NPR Sci-Fi Radio Theater: Dramatizations of several well-known sci-fi stories. Nothing spectacular. Phillip K. Dick's "Imposter" would probably be my favorite of the lot. The only episode I haven't heard is "Close Encounters with the Deity." It's usually mislabled. If you run across it please let me know.
NPR did produce a couple of interesting adaptations of "A Canticle for Liebowitz" and "Star Wars" back in the day. Not exactly the definitive versions, but interesting.
Mercury Theatre on the Air: The War of the Worlds (10.30.38). It's a classic for a reason. Orson Welles' brilliance on the radio can never be overstated. The H.P. Lovecraft Historical Society does a fun homage to this combining it with Lovecraft's "Dagon" that I'd also recommend.
Seeing Ear Theater: This show was a mix of genres, but they did some sci-fi stuff. Good production values, but I'm not a big fan. It's a show that always seemed to zig when it should have zagged. They would have an episode with good actors, but with a bad story, followed by a good story with bad actors. Stories that should have been serious were turned into comedies. Stories that were supposed to be comedies just weren't funny. I'm not going to totally dismiss everthing they did; if you see a story that you like in their catalog it's probably worth a listen. Neil Gaiman's fantasy story "Snow Glass Apples" was probably their most successful adaptation, imo. I also have a soft spot for "The Moon Moth" but it's very silly. The only episode I haven't heard is "Lay Your Hands On Me." If you run across it please let me know.
Lights Out: Old fashioned horror whow. They're hokey and dated but I still find "Revolt of the Worms" and "Oxychloride X" fun. They both have some sci-fi elements. "A Knock at the Door" and "Mungahra" are better episodes but they are strictly horror.
X Minus One: You are already familiar with this show, of course. FWIW my my favorite episode is Lulungomeena. I also really like The Cave of Night, Tunnel Under the World, and Hostess.
Dimension X: "With Folded Hands" and "The Castaways" are both great. I also like "The Embassy" and "Mars Is Heaven." In fact, I think this is my favorite adaptation of "Mars is Heaven."
The Mysterious Traveler: This is a very old radio program, and unfortunately most of the episodes are missing. It was a mix of genre's and they occasionally did a sci-fi story. The one I'd recommend is episode number253 "S.O.S." It doesn't really seem like sci-fi, but, well...you'll see.
BBC: They've produced a lot of sci-fi stuff over the decades, of course. Being an American I haven't had access to a lot of it. I enjoyed their adaptation of "Do Androids Dream of Electric Sheep?" as part of their Dangerous Visions series. It's a little like the novella and a little like Bladerunner and different enough to be it's own thing. If you like Ray Bradbury the BBC has adapted a number of his stories as standalones and as part of a show called Bradbury 13.
CBS Radio Workshop: Check out the first episode "Brave New World" from (01.27.56).
Other shows: Future Tense, 2000+, Exploring Tomorrow, Tales From the Otherside, and the South African program Sci-Fi '68 are also possibilities. I've never listened to enough episodes of any of them to form an opinion. My initial impressions weren't very positive though.
Edit: There just isn't that much really great radio sci-fi, imo. Hell, there isn't that much really great sci-fi period. If you run across anything good, feel free to pass it along.
