My favourite memory from WoW would have to be in cata, when I was near the PvP vendors and was doing random bgs. I was full geared and had no guild, so someone asked if I'd like to do bgs with them and invited me to the guild. I met my partner from those bgs, and it was my first guild I'd been in that felt like a community.
Another memory was when at the start of WoD, my partner and a friend of ours joined a new guild to raid in. Most of the people in the guild didn't know each other, and we were a 3 night raid group, however we ended up raiding for 5 nights in a row when Highmaul came out be