Morrowind still number one in my list. Unmatched open world in terms of imagination, art design and atmosphere. Unrivaled lore, it is pretty much a complete setting and I do include Kirkbrides stuff and even out of game lore in it because it is canon and a part of the universe. Anyone that disagree read the entire arcane library and come again. Great spreadsheet character creation and progession with as much or little autism as you want. All the tools to make your character a peasant or a god, with plenty of a world to live in, fleshed out factions. Uses dice. Good quests that doesn't do meta crap like magical compass markers but lets you be your character and people will provide you with actual directions to interesting areas. Great crafting and many equipment slots. Basically its the best sandbox rpg ever made.
Too bad it's totally static, which ultimately made me develop a hate/love attitude to the game. Morrowind with a working Radiant AI and factions system would have been fantastic.