Well, in my opinion The Last Lore Keeper isn't the hardest boss in Tibia. Actually I defeated it many times and we have never failed with my team. Worth to mention that in this team were players with 500 level, which is not high nowadays.
If we are taking into account global bosses which may fight with whole server the strongest still seems to be Gaz'Haragoth
. His deadly UE till these times may kill many players at once.
Considering quest bosses I would say: Goshnar's Megalomania
. One of the newest bosses from The Soul War Quest, where firstly you have to face mini bosses which are really hard even for 800+ level players. Megalomania is kind of combination of all the tactics known from the fights before. It's a boss with hard mechanics and really huge power.
For now there is no other boss stronger thant these ones. Each update brings un a new content, so probably one day we will face to even worse evil power. I will try to update this info then! :)