I only intended on a short reply, but the memories came flooding out when I started typing. Do forgive me!
I think I would have to say my fondest memories of UO were the glory days of being a Trinsic Borrower on Atlantic... Before Med, when you could have the template Magery, Archery, Resist, Tactics, Steal, Snoop, Hiding. Not only was it a full thief temp, but also a perfectly viable pvp setup. Looting homes, killing the owners and their friends as they tried to lay siege to their still being ransacked house. I am one of the few players left I think who not only remembers the old noto system, but also thinks of it fondly. Being Dasterdly and thus red, putting on a death robe to cover my plate and recalling into the healers to lay a smackdown on the lame notokillers. (I might have played mostly "evil" chars, but like a lot of similar players, I was more honorable than most Great Lords)
Anyone remember those days before spell delay? When you cast a spell soon as you tapped the button? I wasn't playing UO after the AoS release, but I read about the 4 FC mages and it reminded me of those early days when we would machine gun fireball players walking from Brit to Skara... (I remember failing a fireball at master magery! heh)
I also remember fondly my time in The Mercs... Negative 127 noto, with no thoughts of anything but. Remember this temp anyone? Magery, Resist, Archery, Swords, Tactics, Parry, etc? Turning your fellow player into a pin cushion after you blew your mana? I even miss how slow mana regen was... Atleast than a player couldn't be up and running and back in the fight within 1 minute of dying.
Lastly... I think my true fondest memories are just of the community and the game itself. The taverns (Silks on LS! There has to be someone who remembers Bubba and his IB crew!) PR towns, counselors when they came about, No skill and stat locks (This in hindsight I now believe was one of the worst features to be added), When even an adept swordsman was worth something, The naive attitudes of players, we thought anything and everything was possible and we attempted it. The noto system, The virtue guards, Horse Stam (Another change that shouldn't have happened) One land overfilled with tents (Sucked back then, but prolly better than todays multiple lands and waterfall box homes) The websites and comics, Seers and the small quest they put on for as little as one player, 100gp on a new char and having a dog whip my butt upon trying to fight em! Waiting and really beliving designer dragon when he said Necromancy was to be released soon! Getting really excited with the introduction of the original necro regs! The old Context menus (They might have been tougher to use, but once again in hindsight, they were better), the old grey text on everything, item id wands and an unknown magic katana... The old dugeon spawns (I think Hythloth is the only one who has stayed the same in regards to type of monsters that spawn), Pre-runebook and them being blessed (When the rune library was a center of the community) Lord British and Blackthorne (Before the latter went all robocop!), My tamed ancient wyrm! Notorious Npc (If you weren't around then, this surely won't make sense), GM's selling castle deeds, GM Darwin pking players! The UO Volunteer Scandel! The nickname "Smurf", Gating Monsters, Area effect on trapped boxes!
Man, I could keep going forever. I think you get the idea though. These are what I consider my fond memories of UO!