For me its:
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Ultima Online
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Final Fantasy XI
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Ragnarok Online
UO was a game I just recently discovered and it's fantastic, best sandbox experience I've ever had in a game.
FFXI was another recent discovery. It took the formula from Everquest and made it better. The game is even less grindy today with the QOL changes.
RO was a big part of my teenage years. I love the look of the game, it reminds me of Final Fantasy Tactics. It's a great casual game to just pop into and grind out some levels or explore.
Yeah it was a great time for a young teenager to play free games. I use to search for free MMORPGs all the time and there were new games like every week lol.
R.O.S.E, Flyff, lineage 2, etc etc