Now that's a tough one, because the wonderful world of fangirling has gifted me with so many memorable moments!
Meeting Jensen for the first time - that was pretty memorable (and overwhelming). Meeting Jensen the second time wasn't bad either :)
Sharing a flight with Jared to Rome last year and having him remember me from A12, I think I squeeeeed for England after that!
My first con - Asylum 7
Getting Hi-Fived by Misha and then Jared the following year
Making Travis (Ghostfacer Harry) laugh out loud at our meet and greet
Getting hugged so hard by Ty that the photographer actually looked concerned for my welfare
And many others ...
They were all amazing moments that will live with me forever, but I think the most memorable part of fandom for me, has been all the great people I have met, first on line and then in person. So I think a special mention has to go to my very first fangirl meet up with a very sweet lady called Viv (my convention buddy and roomie). A few months after I posted my first story on FF.net in 2010, I was having a PM chat with Viv and we realised we only lived about 15 miles apart, so we arranged a meet up in Windsor one evening for a pizza.
I was a bit nervous at first, (I'd never done anything like this before and I felt like I was going on a blind date), but when we met up, we hit it off straight away. It was Viv that talked me into going to my first con, and there, as a bewildered newbie, I met a lot of fabulous people who until then had only been pen names to me, and who I now count among some of my closest friends. I think it's fair to say I was bitten by the bug and I've been a regular co-goer ever since. That immersed me deeper into fandom, and in doing so allowed me to continue meeting all the fabulous fandom friends, both on-line and in person, I have now.
I don't want to name individuals because I'm bound to forget someone, but you all know who you are!
The simple fact is that I doubt I would ever have gone to a con of my own volition because I had no idea what they were all about, and even if I did, I had no-one to go with; so I wouldn't have met so many people, so I wouldn't have been so active in fandom, and so I probably wouldn't be sitting here answering this meme!
Therefore, I don't know if it qualifies as most memorable or most significant, but I'm going to name that innocuous night in Pizza Hut, Windsor with Viv as the most special thing that happened to me in fandom!