Oh man, this takes me back. I'm going to share my favorite GW2 memory, because I can.

I was playing my main (a thief), wandering around some zone or another. I happened upon a cool looking elite and tried to take it, but it was too powerful and I had to retreat. After I did so, I noticed another player arrive and also go for the elite, and decided I could jump in with him and get the loot too.

We killed it, I don't remember the loot, but what I do remember is the strange portal opening...

We enter, and are teleported to a bizarre other region. We regroup, and spend some time exploring before accidentally finding a portal out of the place and being teleported to random spots in the region.

We immediately /pm each other and regroup at the elite, both eager to find what's really in that other place. Cue 2+ hours of exploration, solving jumping puzzles, and just camaraderie with this stranger I'd never met exploring a beautiful and mystifying area. Finally we reach the prize, a big chest of loot and an achievement, and teleport out.

I never saw them again, and I don't need to. It was such an amazing experience, just raw human connection and exploration, happening by pure coincidence in a game I played exclusively solo. I enjoyed a lot of things about GW2, but that one memory will always stick in my mind, and I haven't found an experience like that since