It seems that TibiaWiki states Pre-6.0, anything before November 3, 2000, contained the following: Deeper Fibula, Ornamented Shield, Short Sword, and the Thais Lighthouse quests. Which means these quests could've been the oldest quests. The 
Winged Helmet previously spawned around every 6 months during a map reset with some gold coins in the Thais Lighthouse where the Dark Shield quest is now. Also, there's a rumor that one of the 
Magic Longswords was hidden by a map editor in a body under a fire field in the Ornamented Shield quest and a player by the name of Taghor received said sword. I started to play Tibia in 2004 and I also wasn't the one designing said quests so I couldn't pinpoint which quest was the first. I do know that Rookgaard came after Thais so I'm a little bit hesitant to list the short sword quest as it's located in Rookgaard. However, it's still considered one of the oldest quests. I wanted to paste to you some interesting quotes from articles upon my research-
"The more random and abstract a secret gets, the less entertaining, engaging and solvable it becomes. In Tibia's history, we tried out quite different approaches to that topic. The first quests were somewhat trivial. The world was incredibly small back then, there was nothing but Thais and some land around it. So with fairly few things to explore, quests relied on some more or less well-hidden switches and keys. The loot was only available every few hours after a map refresh but eventually, each of the few players on this single world had the same collection of items. Even in this very limited world, it was already word of mouth that told people where to go and what to do. So the benefits and drawbacks of sharing information already existed back then, but on a much smaller scale, of course.
Then there was a time with a heavy reliance on first-time explorations. That was still back in the earliest days of Tibia when we only had one game world, mind you. The idea was to only reward the first player who reached a certain area while people getting there later would receive nothing - first come, first serve. However, this era was haunted by problems coming from another source: The server was quite unstable so there were numerous crashes. For instance, you could actually "kill" the server by using a distance weapon on a character that shared the same spot with you. A crashed server led to a reboot and the map was completely reloaded, too, so the treasures were obtainable once again despite the original idea of unique rewards. Only the unpredictability of when a reboot would take place (it could be days) prevented the worst. Yet, as you can probably imagine, the 'forced respawn' of treasures was quite commonplace - actually to an extent that people rather kept the bug a secret to abuse it. The implementation of the magic system made it even more flawed due to the imbalance between vocations, allowing certain vocations to get treasures with no risk at all by using invisibility to avoid monster fights."
This article makes me believe that the items spawned like the Winged Helmet in the Thais Lighthouse were considered the first quests.
Sources:
https://www.tibia.com/news/?subtopic=newsarchive&id=2665&fbegind=28&fbeginm=2&fbeginy=2001&fendd=29&fendm=3&fendy=2020&flist=11111111
https://tibia.fandom.com/wiki/Updates/Pre-6.0
https://tibia.fandom.com/wiki/Winged_Helmet
https://tibia.fandom.com/wiki/Magic_Longsword
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