Half-Life 2 for me. I started first year of university a month or two before it came out, I was in dorms and because the place was newly built it didn't have internet installed for everyone yet and we'd been promised super fast ethernet in every room. I spent every 2nd day talking to management for the dorms and trying to make sure it was in place for HL2 release date, since I had heard that the collector's edition disc version I was going to buy would still require more files to be downloaded after install (That turned out to be partially true as there was a patch but the file verification took the place of a download in terms of time taken).
The release date came around and I had built a new PC recently so some friends were also coming over to watch/play it with me on launch day, so a lot was at stake (in a nerd way). We read in the morning that it needed to "unlock" the files installed from the disc and that it might take some time because the servers were being hammered. We thought that since it was "just verifying the files" we could use a (slow af connection, this was 16 years ago) mobile phones hotspot to get a connection temporarily.
After we realised it would take over a day to verify the files that way we switched tac. We knew other people in the living complex had internet and we also had a 30m long ethernet cable. The place was divided into many separately accessable buildings, which were terraced in groups of 4-6, after a quick check of our own building with alcohol to bribe people to loan us their internet via cable we turned up short. The people below us didn't answer but we just thought that it was empty.
We eventually found someone in another building willing to lend their internet, but the buildings were physically separate. We got people at each window and threw the cable across to the other person, it was long enough but it was really tight and there were insanely dangerous tripwires throughout our own and the main hall of the building. With a better connection the timer reduced to something like 6 hours and we went off to do other things (Some of us just read the initial impressions of HL2 while we played other stuff on my PC).
It got activated eventually, took longer than 6 hours definitely and we started. For this first day we decided to have everyone play each chapter of the game, so there were 4 of us that wanted to play that meant repeating chapter one 4 times, then chapter two 4 times, etc. I can't believe everyone agreed to that haha, but it was the only way to ensure no one missed out experiencing some new elements the game had, like the physics puzzles or scripted events.
It was really fun and we got to the end of Ravenholm by that evening and stopped there. It was great seeing each different person tackle the situations in new ways, with the prior knowledge of those they'd watched play it before.
