Recs.
Updated
The notorious bastard child of John Romero, Daikatana is a classic single player arena shooter. It consists of four different time periods, and includes four sets of weapons in addition to enemies, levels and moods. Assuming you loved games like Quake 2 and Unreal, this will suit you definitely. Nintendo 64 version is still bad though, we don't talk about it.
SpecsUpdate
Pros
Pro Has a great single player campaign
It is both challenging and versatile. The lore is simple, you're a time traveler and as such you will come across different sets of weapons from different eras. The story might be a little cringey at times (has a few tediously long cut scenes), but if you enjoyed Quake 2 then you will enjoy this for what it is as well.
Cons
Con Companion AI is bugged
Sometimes they get stuck or lost, which will prevent you from leaving the level/area. This has been patched in the 1.3 update where you can disable companions altogether, although this needs to be manually updated.