Most of the bosses will give you a chance to drop an rare item that has a high value. The only exception to this are the gold token bosses that drop 4 gold tokens each and 12 gold tokens at the final boss.
The most profitable bosses are:
1 - Oberon - This is the boss that drops the Falcon Set that are by far the most expensive and desirable set for high levels. It's easy to kill but require a small team to access its location safely.
2 - Scarlett Etzel - This boss drops most of the cobra items. It's very easy to access and kill.
3 - Urmahlullu - This boss is easy to kill and you can find groups to do it easily. There drops are not so expensive as Cobra or Falcon itens but there are way more common to loot.
4 - Grave Danger bosses - This bosses are strong, i recommend a team of 400+ to kill them. The best lootable item is the terra helmet.
5 - Ferumbras Mortal Shell - there is a small chance of looting Ferumbras' Hat And mystery Scroll, both are very expensive itens. Even without luck everyone at the team will earn a nice amount of gold. (This one can be done each 15 days)
6 - Jaul - Great boss to farm with a small team. But it depends on a world change.
There are other bosses from Forgotten Knowledge and Cults of tibia but I do not recommend farming them if you don't want the rares for yourself. Remember that you'll take time to kill these bosses and I think that the profit for hunts plus the exp (levels) you'll get are better.