For me, it was when I GMed Magery (18 months of training!).
Second was GMed Healing (26 months of training!!).
Disclaimer: During those times, I played UO for over 6 hours a day, and double that on weekends.
Draconi: I have too many
I think, the top 3 of all time are:
1) The first play my acting guild performed
2) My first player kill (I still remember the spot, northeast of Vesper, self defense! Heh.)
3) The first time I actually helped a newbie get something done that made them want to keeping playing UO Sappy, but it was all warm and fuzzy.
So what was your proudest day??