It is a hard pick for me, because I would be incline to pick multiple additions..... but personally, if I had to stick with only one, I would say it was Treasure Hunting....
Sure, it has its highs and lows over time but as something freshly released, Treasure Hunting was a WOW for me......
What had you say WOW when it was freshly released in Ultima Online ?
I would probably say Imbuing, though I feel it wasn't designed correctly (too much power, made everything else in game obsolete) and the whole SA expansion. It was a lot of fun, it really changed the game (again, possibly too much) and the content lasted a long time.
Edit... after reading, I take it back.
1) Siege perilous. Wouldn't be playing UO at all without this addition.
2) Custom houses. I don't customize houses all that often, but the ability to do so allows you do to so much. It's great to tell a story, let people know who you are, use your creativity...
3) Imbuing.
4) Stealth. I really love stealth, and it was super cool when it first came out. Oh yeah, it also came out in a publish, for free.
Honorable mentions (no longer in game):
Scenarios: I really miss this style of content addition... I wish they would bring it back.
Worst:
1) Tram. Killed the game on regular shards for me. Terrible choice.
2) Insurance. Kicked the corpse.
3) Refinements. Just... just no. The design choices, not the concept. So stupidly implemented.
4) All art post ML. I dislike almost all of it, and downright hate a good deal of it. So much of it looks like it was made without using UO as a reference at all, with no concern as to whether it fit or not. From SE weapons that are all but invisible on the avatar (bokuto I'm looking at you) to all of the 100% perspective circular items (drives me insane), to the nicely done, but clearly higher resolution and thus out of place recent additions, the likes of which you see on holidays. Yech.
Dishonorable mention (no longer in game): Power hour. What a cluster****.