Quake: Marcher Fortress

While quite dated in 2005, was also quite early for singleplayer Quake 1 mapping.
Marcher Fortress impressed many with its Q3stone/metal-styled fortress of epic scale, with a few new monsters in lots of hardcore horde combat (almost 400 monsters in total) as well as its many other custom gameplay additions.
Marcher Fortress impressed many with its use of curved surfaces, as well as its vast landscape similar to the Halo games.