Maybe this is carried by nostalgia in my mind, but this was easily my favorite MMO ever made, and I still sometimes play the fan servers that still exist.

The concept is so well executed, and it had so many quality of life mechanics that didn't exist in other games at the time of its release.
The expansions only further improved this game, and to me it stays the best PvE MMO to ever exist.
On top of that, the community was mostly friendly and welcoming, and I cherish the Task force races, costume contests, raids (Even when Kronos bot ambushed) the massive ordeal that was spawning Caleb in the Nerva Archipelago, and all other great community moments.
Eventually they even added tools for players to create their own missions and quest lines, and this made community involvement even better. Not all of them were gems, but some were absolute gold involving long time members of the community and really allowing people to become a part of the game world they played in.

The PvP was pretty lacking, but that is truly the only thing I can ever hold against this game. Even then though, most PvP zone ended up just becoming excuses for the heroes and villains to work together until that became a feature in a future expansion.