Facing-worlds, used to play it for hours when I was a kid... 15 years ago Instagib + Fatboy +,low gravity +,jump match fun times.
I think the first map i ever played was ctf-eternalcaves, i remember the buzz of running with the flag and got my first cap vs novice bots, i thought you took both flags at the time never played ctf before.
The ones that hooked me on the single player were many, pretty much most of the ones where you can spawn big monsters in. For the multiplayer, probably CTF-Bedrooms and that CTF with sharks, surfboards and the music Surfin' USA, on Roseanne's BOOM.
DM-Morpheus This was the map I first saw UT in, and it's the map me and friends played most of the time. Good times.
Also, I was in awe the first time I saw CTF-Face. Especially because the first time I spawned in it, since the pathway in the middle goes up then down to the other tower, my first perception was instead that the map was much bigger than it actually is and it seemed to me that the other tower was quite far in the distance, and you would need to run the entire pathway "up" until you got there.
It's quite silly, but it was my first impression and it stuck to me to this day. From there, CTF-Face felt quite disappointing. Still a good map to test things up though.
For me it wasn't so much a map but more of a weapon or game type that peaked my interest...Sniper Rifle... Then later I found "The Redeemer" and ended up building a Redeemer server. Basic stock maps that came with UT99 didn't really fit the larger long range weapons...but if I had to pick a map/maps that helped along the way I'd say DM-Morpheus or maybe CTF-Facing Worlds.
DM-Peak. It wasn't my favorite map out of the box, but it was the one that first showed me that UT was going to be something quite special. CTF-HallOfGiants was a later one, and of course, Facing Worlds.
Frankly, there are so many original maps that blew my mind...Morpheus, Deck16, Liandri, Phobos...all of them were quite impressive.