Oceania RO (from before it went sub) and then many, many, many private servers after that.
Anyone remember WaffleRO before Waffle ruined everything? My mates and I were a Super Novice guild that chilled out in front of Payon Dungeon, taking out anyone dumb enough to try and fight us.
No seriously. We just sat there talking to people, and whenever someone started throwing attacks at us, one of us would just stand up and “Frost Diver + Lightning Bolt” them to death. There was this poor Mage who kept trying, then eventually came back as a Wizard to try their luck again only to be once again ravaged by our glorious instant-cast 
… Really did help that Waffle gave us items that actually boosted HP and MP to bring us up to everyone else. Without those, we’d have been one hit wonders.
Edit: Oh gods. Speaking of private servers. Anyone remember the Pinoys and their angry cries of “asa” whether they won or lost in pvp servers?
Edit 2: OH! And War of Emperium. A rare MMO guild-vs-guild that actually mattered. Taking control of castles, upping the defenses so you could keep access to castle specific dungeons… AAARGH! NOSTALGIA!