Treasures of Tokuno is my favorite event in UO. It can be played by any combat character at any level. The scope of the available area is large and no small group dominates the resource (minor drops) for all others. Although it is probably time to update the minors and majors, I hope ToT4 comes and they incorporate this system into other areas.
The Doom drops, UOML Named Monster drops, and Anti-Virtue dungeon drops are also good systems. My concerns with Doom is that access is limited and you are pretty much forced to have a party to survive the gauntlet. There may be some characters that can solo it, but the time it takes makes it hardly worth the effort. The UOML and Anti-Virtue systems are good, but the drops need a revamp. If I could trade in 10 anti-virtue or UOML drops for an item that I select and is useful for my character that would be awesome. I know Sakkarah mentioned in a recent
FOF that they planning to revisit artifact drops. I hope she and the team get to that priority sooner rather than later.
-OBSIDIAN-