I'd have to say the best moment I ever experienced in XI was, when sitting down to think of my best moment, I realized that all of my best moments were with my best buds in THC, and it had been like that for the last 3 years. I fucking love all of these guys. But a few good highlights:
This one time in yug(tab) jungle, when Mokk and I were farming overgrown roses for Laquer tree logs, because that seemed like a good idea at the time, I decided to see if I could tag a fishman off that cliff overlooking the Norg cave with my bow. I could. And low and behold, about 7-10 minutes after I hit 'im, we had a fishmen on our hands. So I do this a few more times (1-2) and Mokk sees if Chi Blast could hit something down there too. So he tags a monkey. However, it seems like the monkey isn't coming, so we grab a rose and a few goblins. Then the monkey showed up. Then his friend. Then EVERYOTHERMONKEYBETWEENUSANDYHOAT. about, I think, 17 monkies with the path it took. It's all fun and games til you get your ass stomped by a horde of monkies.
Waaaaaay back in the day, back when I was still a THF and Darklion was a WHM(years ago) and we we're dicking around in the Maze of Shakrami, and, for a reason that escapes me anymore, we made the ultimate leech train in the Leech King room. And we were just mid levels at this time, so that's alot of leech (I think I was level 30ish) and somehow, we killed all of them. At once. Was damned impressive at the time, and still amusing as hell because of the little *pop* sound they make when they chase you.
...and I have about a thousand other stories like that, but, I've gotta run to lecture.