Ultima Online in the age of dialup and early DSL was a history-altering game for MMO's. I still log on to UO Forever servers every once in a while (a modified game server with accelerated skill learning) to level up a character and scrabble together enough gold to build and furnish a house. It was such a vast game and the sheet depth of it for a game of that time was unprecedented. I still think to this day it is one of the best "just go do what you want to do" MMOs out there, something that New World didn't even get right 20 years after with considerably more resources and advances behind it.

Runescape is the only other game from that era (and which shares a lot of gameplay similarities) that has that much staying power.