No offense to all the native mapping talents at TWHL, but de_nightfever_beta by d00ds is one of the best custom CS:S map I have ever played.
Not only does it look amazing (perhaps one of the best use of lighting and lightmaps in any map), it plays extremely well too. The layout is wonderfully fun. The attention to detail is astonishing.
This is only a beta release, so there are still some minor errors (including a no_draw glitch and overlapping texture), but they are pretty small mistakes.
Make sure you check out the view from the courtyard on the roof and take a ride in the elevator.