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I would have to say that Ultima 7: Black Gate was the guide. My experience in that game is what lead me to look for online multiplayer games literally. A search for a mod to play multiplayer is how I found UO. I love the NPC that had lives and made me wonder what a game would be like with everyone living out there lives online.
UO would be the number one MMORPG. I expected all games to be like UO after playing it. A wonderful virtual world. This was what set my taste for Sandboxes. It was flawed but it was fun.
EQ is a mixed bag for me. Back then it seemed like a step back to just a leveling treadmill game. I was not enamored with the game like a lot of people. But I've come to appreciate a lot of the things it brought such has cooperation, factions, lore, and challenge that are lacking today.
I would say Horizons. The original vaporware version not the Istaria: Chronicles of the Gifted it became. Back then I used to follow games with a naive gleam in my eyes and dream of these ultimate games that would never be. It was bringing all that was UO together with the best of EQs races, lore etc. The game was supposed to have craftable player events to unlock content I guess similar to EQ:Next Rally Calls. Dynamic invasion from monsters controlled by DM. Races that were supposed to be always at war. Rare races that were only obtainable by rolling for them at creation. Player built kingdoms and deep crafting. Never got anywhere near that though.
Lastly, I would say WoW. WoW simply for its presentation of UI and quest. Its polish and refinement were unmatched at release. It's the ground level of what you expect for a modern MMO to play like.