It was a bit late in EverQuest's prime, but for me, nothing has ever come close to
The War for Firiona Vie, and the massive events leading up to it.
It was the biggest Good vs Evil battle I've seen in an MMO.
It had all the big EQ characters participating: Lanys, Al'Kabor, Firiona, Galeth, Ykesha, and more I can't remember.
The entire event went on for a couple weeks. Starting with the Trolls waging war on Gukta, and retaking Grobb. Shortly after, Dark Elven and other armies began amassing outside their cities. This led to hordes of baddies marching on good-aligned cities all over Norrath, and even Luclin. They tore through almost every zone in the game, with many bosses with amazing and unique items up for grabs.
Eventually about two weeks later, the War reached the Dreadlands, where the two armies went at it for hours. Shortly after, it extended into Firiona Vie, where the big bads waited to be killed. One even dropped a sword modeled after Seru's Sword of Truth. After even more hours of fighting, the main outpost was eventually taken over by Lanys and co.
It's just something I'll never forget. It was the pinnacle of EQ for me.
At the time, there were massive writeups on each stage of the event from many different players, though there seems to be very little information about this event floating around now.