Back in the day, my buddies and I used to play UT99 all the time. We were mostly using our craptastic 33.6k modems, except on rare occasions when we had access to a LAN environment, but it was still great.
Prior to that, we used to play Quake 2 and Duke3D, but for some reason, we could never get into Quake 3. To this day, I don't particularly like that game. Somehow, the physics of Quake 3 feel weird to me, the sound seems oddly muddled, and the weapons just aren't satisfying to use. In contrast, every single weapon in UT99 seems like it has a distinct purpose, and the cool sounds that they make are ingrained into my mind to this day.
EDIT - just to clarify what I mean about Quake 3 sound being muddled. Positional audio using A3D 2.0 is excellent in that game. Probably one of the best uses of Aureal's Vortex2 hardware, and I have zero issues with that. It's the sound design that I don't like. In particular, the sounds that Quake 3 weapons make when firing and hitting opponents just seem weird, especially compared to how awesome everything sounds in UT99. Might just be personal preference.