Legendary Sandbox MMORPG. You are dropped into an immersive world to make your way as your choice (or combination) of a crafter, warrior, wizard, archer, beggar, thief, murderer, fisherman, drunk, animal tamer, merchant, assassin, bard, fashionista and on and on… You define your journey and how you will interact with the world. Will you strive to amass wealth? Will you fight for good? Or, will you fight for the forces of evil? Will you fight other players? Or, will you venture into dungeons to take on the demons and beasts within? No pre-defined quests or story paths to follow. Everything is customizable. Still the only game to have ever gotten player housing right. Fully customizable houses that you can decorate as well as a virtual real estate market. All things considered, the best game I have ever played.
I will close with a quote from Star Wars that also describes Ultima Online perfectly, "Mos Eisley spaceport (UO). You will never find a more wretched hive of scum and villainy." - Obi Wan Kenobi