Ultima Online
It was my first MMO and I was 13 when I first played it in '97, so I might be biased, but nothing can come close to the level of player-freedom that game allowed. It felt like you could do anything. Want to own a large tower but no longer any land plots that it will fit in? Murder the owner and take his keys. Want to spend 90% of your time at the smithy in Minoc or Britain and just make wares for people? You can do that too. I could spend days just chatting about all the stories from the game and whats interesting is none of it was from any scripted event within the game; it will all player-driven.
So here is a bit of a story from my time with Ultima Online and one instance of the game that will allows stick in my memory:
When factions were implemented in the game, you basically had this giant 4 way capture the flag going on where each faction would attempt to hold onto these sigils for a period of 24 hours (or some similar time period) to control specific cities within the game for a period of time. I was a part of a faction called the True Britannians initially and would spend entire school-nights staying awake defending our castle in Britain (one of the major cities in the game), which was our faction HQ. Over time I decided for whatever reason that I wanted to change factions to the Council of Mages, HQ'd on the island city of Magincia. When I made my split with the True Britiannians, a friend of mine took it VERY personally and we became bitter enemies over the course of the following months. Eventually our faction was able to get a hold of a number of sigils and two of the factions, the True Britiannians and the Shadowlords, both layed seige to us to take our sigils.
What was interesting about each HQ was that each one was distinctly different. The True Brits had a castle with limited access, Shadowlords had a crypt to defend with a very small area to enter from so it was super easy to defend, and the Council of Mages was on an island but there were a number of ways it could be attacked. There was a moongate for people to enter from on the north part of the island, plus you could just recall in the game to most areas, but once on the island you could only enter the HQ via a bridge on the southwest portion of the island. What made our HQ easier to defend was that while you could not use magic to recall into our HQ, you COULD use boats to bring supplies in. Sieges would often become deadlocked and in Ultima Online, you had to us stamina to push through other characters and you could throw down random objects like boxes, chairs, etc to form a wall of which people could pick these items up but you had bag space/weight limits, so it was always a valid strategy to attempt to obstruct entrances. However when these deadlocks occurred, it came down to who could supply themselves the longest. Obviously the ones laying siege had the advantage in this regard, as they could just resupply as needed an the those defending were usually stuck in their HQ, hoping the attackers would wear themselves out and eventually log off or you would cap the sigils. So it was a game of attrition most of the time.
So the siege was underway, and we were holding strong for a number of hours. The TB and Shadowlords pushed onto the bridge but could never make any substantial gains. We had mages placing poison walls up and any other number of obstructions, but as the hours went on and the morning went into the afternoon, their numbers became larger and larger. We had to make supply runs with ships for arrows, bandages, reagents for the mages, and more and it was a hell of a lot of fun coordinating that. I would say the numbers were about 40 to 50 of us in the HQ defending which 100-130 attacking. Server lag did occur but it was never too bad. Eventually though, our numbers were falling and their numbers grew, and they pushed through the bridge and were at our entrance. Until this point, I was using my bow to attack them but I also had Grand Master fencing and was eager to mince up our adversaries face to face. Another hour or two went by with us holding our ground but since we were all engaged in combat at this time, nobody was making supply runs and eventually the inevitable happened...they pushed through and we were making our last stand within our base.
My faction, the 20 or so of us left, we knew were going to die. There was no hope of victory for us this day. I applied deadly poison to my blade (deadly poison would kill people in 3 to 4 seconds if it gets applied to someone after you successfully hit them with your blade), and attacked the closest enemy to me. It was, and I shit you not, it was my friend, now enemy, from the True Brits. The poison applied and I was just in awe, even as I was killed within seconds by everyone around me, to see him die right next to me. It was like something scripted for a movie or something, and it could not have ended more perfectly.
So yeah, I think Ultima Online was well ahead of its time and if someone made a game with the right amount of funding now but similar ideas, it would be huge. Or maybe it wouldn't and thats just nostalgia blinding me but regardless, UO was groundbreaking and ahead of its time in in own right.