CP Sulfur and CTF 2Fort Invasion stand as the only two maps that have ever had me boot them up for the first time and spend an entire hour just exploring the maps with a slack jaw. The great attention to detail, the intense atmosphere, and the sheer grandness of the vistas in those two maps continue to blow my mind. With 2Fort Invasion, it also perfectly captured the feeling of, "This is 2Fort, and the aliens just invaded. We are in some deep, deep ****."
Asteroid looks to be really nice-looking, too, and with a terrific skybox, but obviously it's not finished yet.