Until about the middle of last year, I had little awareness of single player Quake mods. I ended up meeting Spirit from Quaddicted.com and I wanted to be more familiar with single player so I knew the technical issues involved, particular with JoeQuake (which I have historically tried to help with at that forum).
I went to the Underworldfan site and downloaded a couple of single player mods from the "all time Hall of Fame" and without any exaggeration, some of the ones I tried blew my mind.
I had in my head a stereotype of boxy Quake maps like the old skool stuff expansion packs (Scourge of Armagon, Dissolution of Eternity).
These maps were nothing like that all.
Here are some of the ones that are my personal favorites, most (but not all) are consensus "best ever" maps.

Lunaran SP1. A single hightech map that is both beautiful and hard to unravel. It was released in July 2005. Download page.

Marcher Fortress. This enormous map is generally considered the 2005 map of year by most mappers. It will not run in WinQuake, you must use GL and I don't think the original GLQuake can run it. It works fine in JoeQuake and therefore certainly QRack. Download page.

Duke of Ontranto. Released in late 2004, it is really different. It has an ending that reminds one of a Doom-style fight. It was very enjoyable and what makes it different is that it has settings in swamps and underground. Download page.

Once upon Atrocity. A castle map that has an eerie calm to it. The encounters are few and far between, but the landscape is fascinating and the encounters always are shocking despite having a good idea of when they will occur. Download page.

Insomnia. Considered by most to be the all-time best single player Quake episode. You can tell it was someone inspired by Half-Life. Download page.

Cheyenne Mountain Complex. This map was released last year, and while probably not on many's all-time best list, it is a very well designed base map that is quite different from most others out there. Download page.
If high quality single player episodes and maps interest you, the following sites are an excellent source of numerous, high quality downloads.
http://www.planetquake.com/underworld/ (still active reviewing maps)
http://www.planetquake.com/teamshambler/
http://www.celephais.net/forum (Single player released often announced there).
I went to the Underworldfan site and downloaded a couple of single player mods from the "all time Hall of Fame" and without any exaggeration, some of the ones I tried blew my mind.
I had in my head a stereotype of boxy Quake maps like the old skool stuff expansion packs (Scourge of Armagon, Dissolution of Eternity).
These maps were nothing like that all.
Here are some of the ones that are my personal favorites, most (but not all) are consensus "best ever" maps.

Lunaran SP1. A single hightech map that is both beautiful and hard to unravel. It was released in July 2005. Download page.

Marcher Fortress. This enormous map is generally considered the 2005 map of year by most mappers. It will not run in WinQuake, you must use GL and I don't think the original GLQuake can run it. It works fine in JoeQuake and therefore certainly QRack. Download page.

Duke of Ontranto. Released in late 2004, it is really different. It has an ending that reminds one of a Doom-style fight. It was very enjoyable and what makes it different is that it has settings in swamps and underground. Download page.

Once upon Atrocity. A castle map that has an eerie calm to it. The encounters are few and far between, but the landscape is fascinating and the encounters always are shocking despite having a good idea of when they will occur. Download page.

Insomnia. Considered by most to be the all-time best single player Quake episode. You can tell it was someone inspired by Half-Life. Download page.

Cheyenne Mountain Complex. This map was released last year, and while probably not on many's all-time best list, it is a very well designed base map that is quite different from most others out there. Download page.
If high quality single player episodes and maps interest you, the following sites are an excellent source of numerous, high quality downloads.
http://www.planetquake.com/underworld/ (still active reviewing maps)
http://www.planetquake.com/teamshambler/
http://www.celephais.net/forum (Single player released often announced there).
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