Tons of them. Vecna, Bahamut, Tiamat, Bane (? IIRC he's in one of the early 4e Dragons issues, but maybe it's just his aspect or something)... prolly one or two more.
Primordials, Gods, Archfiends and Daemon Princes (some of whom were Gods/Primordials)
Vecna (level 35 solo, Open Grave)
Kyuss (Level 31 solo, Open Grave)
Orcus (Level 33 solo, MM1; Level 34 solo, E3)
Dagon (Level 32 solo, MM2)
Demogorgon (Level 34 solo, MM2)
Lolth (Level 35 solo, MM3)
Allabar, Opener of the Way (Level 30 solo, MM3)
Imix (Level 32 solo, MM3)
Ogremoch (Level 32? solo, MM3)
Blazing Rorn the Fury (Level 33 solo, Forgotten Realms Campaign Guide*)
Graz'zt (Level 32 solo, Manual of the Planes)
Oublivae (Level 30 solo, Demonomicon)
Pazuzu** (Level 33 solo, Demonomicon)
Torog (Level 34 solo, Underdark)
Tiamat (Level 35 solo, Draconomicon I)
Bahamut (Level 36 solo, Draconomicon II)
Of these, Vecna, Torog, Tiamat, Bahamut and Lolth are Gods. Everything else is a primordial, daemon prince or archfiend. With upgrading damage values, I still think that Tiamat is the strongest solo of the list. Also Bane was never statted because he's arguably beyond level 36. Designers at the time during the drama of "Why no stats?!" pointed out he'd be likely to be around level
38. So is in fact well out of the general reach of all but the absolutely best optimized PCs.
*I do wonder how many people even realize this guy exists.
**He is either immensely ridiculous or just ridiculous, depending on how you read a certain power.
Edit2: After a bit of dinner, I remembered there are actually some others I forgot to list.
Phraxas the Decayed (Level 31 solo, Demonomicon)
Kostchtchie (Level 31 solo, Demonomicon)
Turaglas (Level 30 solo, Dragon Magazine 376)
Bel-Shalor (Level 34 solo, Eberron Campaign Guide)***
Geryon, the Broken Beast (Level 30 controller, Dungeon Magazine 176)
Solkara (Level 34 controller, The Plane Below)****
You can tell from these lists that if you like Daemons, you have So. Many. Options. I mean look how many level 30+ epic demons there are! Primordials and Gods make up the rest of it. Personally, I would like to see a
lot more variety here, even if that means going and filling out some Devils. I actually think that Geryon may be the
only level 30+ devil in all of 4E! There are lots of great archfiends that we could have as well, especially if wizards makes a book for devils ala Demonomicon (hint hint).
***Strictly put, he doesn't really fit the criteria, but in terms of being an epic level solo that is a demon who dooms the entire world if he wakes up he is relevant. Also note that despite the time he was published, he's arguably one of the most effective solos in the game.
****I concede I only remembered her at literally the last moment now. She does have one of the most literally hilarious powers ever though, which makes it even odder that I forgot about her. No offense intended Solkara, <3