I preferred Quake but I also played Unreal 1 and Unreal Tournament to death. I thought the single player in U1 was awesome and not just from a technical perspective. What's not to like about finding yourself in a crashed prisoner spaceship on a strange lush green planet where an oppressive alien race is enslaving the cute Nali inhabitants. Discovering more of what's going on around you by reading journal entries you found along the way. Ofc the fact that the game was drop dead gorgeous and sounded great helped in making it even more enjoyable to play.
The MP in UT was superb because of the advanced engine, fast gameplay, great selection of weapons and a near endless amount of maps.
Hell you can still find UT servers online today, I think that alone speaks volumes about the games quality.
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Edited my first post in this thread because I did "scream" decline a post later. Looks like I can't find anymore excuses to hide my disappointment with the lack of modern skill-based shooters