I voted for Tieru and still don't regret it: firstly, he is, after all, a Dragon Knight. Besides, he knew about the demons' conspiracy, healed Agrael, and gave him instructions (which later turned out to be the only correct ones). Even after death, like Qui-Gon or Obi-Wan from Star Wars, he managed to stay in Ashan of his own volition to "guide" Findan, and of course, Railegh. He was also obviously a very powerful elemental mage (creating fog over the entire archipelago is no joke).
In general, theoretically, Sar-Elam should be the strongest, if we're talking about the Overlord. After all, he imprisoned the Overlord and all the other "bad guys" with a single spell.