When I think about it, Morrowind is probably the game that formed 90% of my interests when it comes to both fantasy worlds and games in general. Years later, weird, intricate, carefully-crafted fantasy worlds are like my gaming crack.
And not only was Vvardenfell a huge, varied place to explore, but unlike Oblivion, Skyrim, and so many open world fantasy settings since, it was weird and intricate; mysterious and intimidating; old and lived-in. I still remember making barefoot pilgrimages to holy sites, stumbling across Ashlander tent-settlements, and desperately searching through bookstores to find out why the dwarves suddenly all disappeared. Opening the game case and laying that huge, glossy map across the table to pick out new places to travel felt pretty amazing.
And nearly every little thing in the world contributed to its unique sense of place, from the open xenophobia of many of the native Dunmer, to the way that the cultural encroachment of the empire (as well as the Nords in the very north of the map) is felt in varying degrees throughout the island through something as simple as competing architectural styles. It felt like Bethesda put so much thought into every little aspect of the game, and these details really made the game what it was.
Yeah, it's mechanically pretty broken, and yeah, plenty of games have made bigger, shinier, more grandiose game worlds, but so few have ever had the weird, byzantine glory of Morrowind.