EverQuest, and its first two expansions Ruins of Kunark and Shards of Velious, is not only the best MMO of all time, its also the best game of all time. Successive patches and expansions diluted the forumula, but no other game struck the right balance of risk vs reward like EQ did.
It had the best dungeons - Sebilis, Guk, Crystal Caverns, pretty much every one on Kunark. You felt the scope and scale of the world because fast travel wasn't an option for most. It poineered raiding, it had a complex system of factions that was a meta-game in itself, and it was a true mmorpg and not just another mmo, as its roots in roleplaying were very much evident. It's itemisation produced some of the most memorable and lusted after equipment thats remembered to this day - Manastone, Fishbone Earring, Amulet of Necropotence, Paw of Oppala, Testament of Vaneer, Flowing Black Silk Sash (I could go on for hours, none of it was transitory and you needed a second set of gear for raiding). The spells allowed for emergent gameplay and you could tell a good player from a bad one when they started using them. The spell names were a celebration of the English language - no Fireball I, Fireball II and Fireball III nonsense here. The slow progress and death penalties meant you really needed a group, and this fostered one of the strongest, knowledgeable and enjoyable communities I've been a part of, both on forums and in game.
So there you have it. EQ was the GOAT, a game that had zero focus testing and market research. You often hear developers talk about how "we're making a game we'd want to play" but most of the times thats just not true and we all know it. In EQ's case, it was. If you got to experience it from 1999-2003, congratulations. Because it was the perfect storm and you'll never see its like again.