Oh, there are many, many beautiful maps:
I remember being blown away by the beauty of auh3dm1 back in 2000:lvlworld.com/review/id:614
and lloydmdm2 from 2000:lvlworld.com/review/id:367
Lonely Planet has mind boggling architecture and is also from 2000:lvlworld.com/review/id:83
There is Chartres a year later which probably burned some graphix cards to toast:lvlworld.com/review/id:1624
And these are 'very old' maps.
The development of q3map2 had a lot of influence on more 'recent' maps and the outstanding quality and level of detail you can find in many maps.
From richoi's list: Estatica was one of those maps that displayed the possibilities of q3map2:lvlworld.com/review/id:1555
Special mention of the map lun3dm5:lvlworld.com/review/id:2238
It is just jaw dropping. The screenshot alone is insane....
And this for a game that is all about gameplay and the 'flow of a map'...
I need to mention that there are texture artists which contributed such beautiful textures sets. These sets make some 'simple' maps look stunning too. Like EvilLair's textures and textures from Sock.
And then there is the art of making skyboxes.... (and the beauty of good item placement).
Corrected the link to lun3dm5 with the right one.Edited 16.77 hours after the original posting.