For me its:

1. Ultima Online

2. Final Fantasy XI

3. 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.