Unreal Tournament GOTY (often referred to as UT99, so as to keep in the naming scheme of the sequels) is a masterfully crafted FPS that deserves to serve as a landmark of the genre for all posterity.

Each and every weapon is distinct, entertaining and in some cases deliciously imaginative (the translocator, for example, fires a disc-shaped beacon that the player can teleport to at will, moving around the map quickly... and killing anyone unfortunate enough to be in the way. This allows the player to quickly traverse the map, and opens up Z-axis manoeuvring for skilled players.
The UT99 community was prosperous in its day, and so there is a gigantic array of maps, mods and other goodies for free download, much of it of such a quality
Though the graphics are, of course, dated by today's standards, the game is very pretty by 1999 standards, and both included and user-made maps have some exceptionally beautiful sights.
The AI bots are generally very competent and intelligent, and on the highest difficulties are dangerous enough to threaten even the most godlike of FPS players.
If you want a highly addictive, skilful and beautifully crafted with timeless quality, you would be hard-pushed to find a better deal than this.