For me it is definitely world 9 of hafen before I quit. I can't really go into details because I have so many stories I could literally write a book, and we are all playing the same game so we all know how it is.
Needless to say I've learned a lot about myself and others and actually changed a lot as a person. I've accomplished things I never imagined possible, some I'm ashamed of and some I'm proud of. Some which were easy and some which required all my dedication and willpower. The relationships and experiences I've had in this game whether good or bad, I will remember the rest of my life. I truly felt like I had lived a life as real as my own. What an amazing thing to come from a video game
Honourable mentions are space station 13 and vanilla WoW. Space station 13 to me was all about being a character with a specific, often pointless goal, and trying so hard to achieve it, like infiltrating the theatre closet to steal a monkey costume to hide amongst the monkeys to assassinate the geneticists. Or chef who kidnaps people to feed their human meat to the unsuspecting crew. Being "that one guy" who does the funniest things that people will tell stories about forever. With WoW it was all the things I could do that nobody else would try, like sneaking my noob rogue into the enemy tram and spending countless hours making the entire faction search for me, while others people were just trying to level up or something...Or trying to turn 1 copper into 1000 gold in the auction house. Or trying to get to spots on the map that nobody else would even attempt
What makes a game profound to me is the ability to set myself apart as an individual, without compromising realistic interaction. To achieve my dreams , to create my own purpose and see it through to the end, or at least fail in profound ways, among others who are trying to do the same thing for themselves. I know it's deeper than this and I wish I could express it better. I wish I could show everyone my experiences, I wish I could watch them all again like a movie before the details begin to fade. The things I've seen...
Wow this ended up way longer than I expected. I'm not sure what I'm trying to say so I'll just leave it here..