Putting my fondest memories into words is kinda hard, since there's so many. It's more like a collection of anecdotes, little stories, events, so many things that happened that makes a whole memory.
From the moment we moved from other Linkshells into Dimensional Deaths, the times we invaded Sky, how we learned it like the back of our hand, the times we failed or had to give up for any reason (like Lightday followed by double Light weather... on Byakko). Breaking the Shadowbind, nuking Shiva with Freeze II, Limbus runs...
There's the memes, the PowerThirst, the videos we made, the meetings at FanFest. No matter what we do, winning or losing, it's a memory and a learning experience.
Then DD moved on to other games gradually, until FFXIV appeared, and then we do it all over again. It doesn't really have the same kind of multi-alliance events that FFXI did, but more important is that the people involved are the glue that keeps things together. 10 years later, we're still a community, more than simply a Linkshell, a guild or a Free Company. It could go by any name, it's still us.