Most if not all of my memories are still vivid for me. It has been a year or so yet I still remember every single thing that has happened. Even on my first day.
But I will agree with Marvolo. Khalazdad and I spend a great deal of time together. As I was getting to know him better the events that took place at those times are unforgetable.
Yet honestly, I can not say which memory was more vivid than the other. They all are of great importance to me and the feelings I get if something or someone brings hose memories back, I really have no words for it.
I remember the times I was bartending in the Sozzled Salamander (Wind's Sanctuary), I remember my training days at Willow's, the private moments I spend with Keith Moon and Z, the day I was adopted, my winning on HC Contest during X-mas, my wedding with dst (THAT was priceless), the day my avatar turned golden (I am sure this is also Z's vivid memory), the first time I saw Bob bloom, the day Khalazdad left and many many many others.
Forgive me if I could not answer your question properly with a single answer but it was really hard for me to choose which was more important. Unlike other places, MD gave me wonderful and unforgetable memories. Most if not all, I wouldn't change them for the world.
But now that I do think about it, the most vivid memory I have is the first day I joined the community for it was also the same day that I entered Necrovion. That defined everything that was to come.