How good the buildings are usually depends largely on the capabilities and experience of the builder. With all the gathering of material needed and the lack of worldedit and fly privileges, buildings are usually much smaller than I'm used to in Minecraft classic. The many details and mods (3dforniture, homedecor, stairs+, ...) and lighting allow for smaller buildings to look impressive as well.
On Redcrabs server (might be one of the, if not the, largest out there), buildings are of a mixed quality. If you're looking for good ones go to Desert City which is a well built city with more than one impressive building (spaceship, mall, town hall, cinema, castle, ...). Ades medieval city is worth visiting as well. The Eiffel tower and water pyramid (sadly still partly dammaged due to the mapgen error) are really huge. Unfortionately, they seem too large for my computer (hardly possible to see them whole). There is an even larger structure in the north which is under construction (don't know what it will become). There are some further good buildings/cities underground. And of course Greenvale isn't bad :-) It consists of few houses in diffrent styles (one of them's mine :-)). Link:
http://minetest.net/forum/viewtopic.php?id=1705Another server I'd like to recommend is King Arthurs Land. The server is relatively new. Yet there are quite a lot of buildings already - buildings which are of high quality. The tower bridge is especially worth visiting. Link:
http://minetest.net/forum/viewtopic.php?id=3292Further good buildings might be found on creative servers like that of Landmine. I havn't been there yet so I can't recommend anything special.