The answer is clear: any of the old Novan wars.
It was no coincidence that Nova was known as a place of eternal war and struggle. Back then, when only Antica, Nova and Premia were available, before even Secura was born as the first PVE server, Nova was known to be the haven of Player Killers and power hungry individuals. Why? Antica was a much older server, and it had a more robust core of higher levels. Powerful and ancient guilds that maintained a relative balance, even though wars were not unusual. Many of the old Anticans moved to Nova to pursue the ambitions they could not fulfill in their homeland, and the conflicts that ensued were legendary.
You could say that many of the wars fought since the birth of Nova were spiritual successors of the very first ones, where two very powerful and distinct forces were drawn against each other:
- On the one hand you had La Onda / Ruthless Seven / Demonic Brotherhood: with a core of Swedish players from Gimo and Halmstadt. Wade Walker, Jofiwug, Warchas, Shadwell and many more under them, dedicated PVPers who created a reign of terror. Thais was traditionally the center of their sinister domain. The bodies from innocents and enemies alike would decorate the streets of Thais.
- On the other hand, Justice / Knighthood, a more international collection of professional PVPers guided by a Spanish core of hardcore gamers, led by three brothers: Messiah, the most feared master sorcerer in the history of the server, Breaker and Paca. They commanded great fear and loyalty, and were by no means no less abusers than their nordic counterparts. They made their home in Kazordoon for a long time, and it wasn't rare to see their rune farms in Tunnel Gardens.
Both forces fought under different banners and names, but the enmity between them could only be resolved by the gods. Eventually, given the many rule violations and the unstoppable forces they represented, Cipsoft ended up removing many legendary players from the lands of Tibia forever. Mass deletions and permanent bans made sure they disappeared, but the warring spirit of the server never did so, and thus the legacy of these two rivaling forces lasted for many years.
As it is very hard to find videos or even screenshots from the early wars referred above, I have included some latter conflicts, the Endless Shine vs the White Eagle Novan war, where some of the followers of the two factions above took arms to decimate each other. Enjoy!
https://www.youtube.com/watch?v=FFsTkmQe4Ng