I will say this: there was a trend of "cram more and more space, in an interlocked layout, with curvy architecture, into a single BSP".... Than's base map was a masterful example, Nehahra had plenty of those style of maps, and Marcher with its ridiculous technical standard, really achieved a level of that that hasn't been rivalled yet. That said, I have no doubt that within RMQ there are several maps that outdo that style of map at its own game (not just STARKMON style ambition of scale, but "pretending it's q3" curves as well - which Marcher does). I still don't know how Kinn got MArcher 1 to compile, heh, but BSP2 now does exist. 

Pick one:
http://www.quaddicted.com/reviews/apsp1.html
http://www.quaddicted.com/reviews/apsp2.html

Design+gameplay+layout+technical execution 

wrong game i know, but i still think that SPoG q2sp map ruins all of them (how awesome would a port be?) 

Plumbers is also a brilliant map too ! 

Plumbers is possibly one of my favourite idbase maps. 

i guess as a mapper i see these things more, but i'm always impressed by negke's psychotic brush mashing skills. 

My all time favourite remains Moonlight Assault. I know I'm mentioning this map a lot recently.

http://www.quaddicted.com/reviews/moonlite.html

Visually its not a patch on Marcher, but the gameplay feels so much tighter, until the end battle at least. A real quality piece of pacing, and under id1.

Honorable mention has to go to Insomnia (czg07c) as well:

http://www.quaddicted.com/reviews/czg07.html

Again, for its time its visually good, now, less so.

Marcher's failing was the spiral staircase section - it felt very contrived. Assault and Insomnia suffered from too much openness - not sure what I mean with that tbh. 

To the small amount of information on what inspired Kinn on this as well.

http://forum.wordreference.com/showthread.php?t=489252

Interesting. 

Spogsp1 totally rules, I've lost count how many times ive played that map! 

Missed that post. Yeah, it's class. 

i never played any q2sps... thanks for letting me know about that map. it was awesome. :) 

Need more q2sp recommendations? 

if you have any? are there even places that host q2 maps anymore? 

I've never played any q2sp's either but will check out that one for sure! 

i'd like to check some quality q2sp maps 

first and foremost, get grinspq1-3: castles of stroggos - it's like the q2/tech equivalent of the mexx bishop trilogy. the first one isn't amazing but it's worth it for completeness' sake; the third one is absolutely fantastic, one of my favourite ever FPS experiences :)

some others that spring to mind;

angron2: premonition of angron
psph20: the powershere quest (coop episode, use skill 0 for single player otherwise it's insanely hard)
genome: prognetto genome (bit hordey at times but visually stunning)
du: dark undergrowth
1964com: echoes of the past
strike: retaliatory strike
ddb: deaf, dumb and blind
batf: back at the front

most of those can be found here: http://www.gamers.org/pub/idgames2/planetquake/ltdan/ - google should suffice for any that can't :) 

some of these overlap with rj's recommendations but this is the cream of the q2sp crop IMHO (in alphabetical order):

1964 Echoes of the Past
Amari Aliquid
Awakening (mod)
Back at the Front
Castles of Stroggos (gets better in build quality, ep3 is phenomenal)
Citizen Abel 3
Citizen Abel 4
Dark Undergrowth
Deaf, Dumb and Blind
Last Day on Stroggos
Light (needs awakening mod)
Love Story of the Prince of Stroggos
Machine Reborn
Monumental Misery
Nightmare Orchards
Premonition of Angron
Retaliatory Strike
Spogsp1
Space Odyssey 2
Space Odyssey 3
Space Odyssey 4
The Widening Gyre
What the End is For 

there was some guy of the quake sadlark fame whos been working on episode/unit some time ago. what happened with him? 

sequel to Angron, lost it in HD crash I believe.

The only q2sp still being worked on, or was, was Musashi's map pack :

http://musashi.backshooters.com/index.html

But last update was Jan 2010. Maybe Maric knows? 

http://tenfourmaps.telefragged.com/php/revidx.php?gameid=q2&sortby=award&page=1 

But the one's I'd pick out as special favourites;

Citizen Abel Series
Light (Needs Awakening mod)
The Widening Gyre
Spogsp1

And anything made with the Lazarus mod - even its test maps in a series of abandoned warehouses are fun.

There was one Lazarus map pack, I think it was called Garage Inc - that was pretty good. 

There is one missing off those lists:

Strogg Towers by Daz 

 

Another one you should definitely try is 'Wierd Comic Art Quake2'. 

I used to play Q2 custom maps, and checked almost every recommended one, but I never really liked how Q2 plays.
Overall balance, monsters, slow weapon switching and other timing effect - everything seems off making it much slower and less fun game than older shooters (Doom, Quake, Duke etc)