In my opinion, the biggest theft is that of the Magic Longsword. A Magic Longsword today keep in mind is worth a lot of money, as this item is hard to come by and has so much history. The numbers I'm about to provide to you were considered a lot back in the day so presently they might not seem a lot. Way far back in time according to Tibia Fandom, a player named Hagbard had an auction which was won by Alex for 5.5 million gp. The sword was then purchased for 5.5kk from Siobkerry, of Hokuten. He sold the sword for 7kk to Recca. However there was a war between Hokuten vs Satori (Recca's guild). A safe trade wasn't possible so they decided to use a middleman. They decided to use Alex because he was a trusted trader so they thought at that time. Alex was also Strenni who was apart of Hokuten which Recca didn't know. Siobkerry and Alex robbed Recca their enemy.
There's even some books in Tibia about this situation here's a quote: "This would be known as the biggest theft in all of Tibia, and to this day it stands as an example of what a simple sword can do. Drive a virtuous man to greed; bring enemies together, and, in the end, cause more strife between them than ever before; make a famed trader ruin his reputation and indeed, even his ability to roam the streets safely. All for fame and power." Please, I urge you to look into my sources incase you want to read more about a major theft in Tibia: