The Circle of Three reward casters of the Three better than all other adventurers for this task to help cure Unsar of the plight he has created for himself through his spell experimentation into polymorphic magic.
They do reward other classes, but all other classes are limited to one reward, which is an AA to give them greater inate magic resistance. The task can be repeated, but only for XP.
For the casting classes that are recognized by the Order of Three, known rewards are as follows:
Enchanters have 4 unique reward. Enchanters 25 and under are limited to one of these. Enchanters higher than 25 have 3 other possible rewards.
Helena, Mezmer: Spell: Illusion Iksar (Level 20)
Mezmer, Donna, Helena: Spell: Illusion Imp (Level 45)
Magicians have 4 unique reward. Magicians 25 and under are limited to three of these. Magicians higher than 25 have 3 possible rewards as well, with one of them being limited to higher levels.
Wynu, Reanna: Spell: Monster Summoning I (Level 30)
Spock: Spell: Summon Silver Choker (Level 21)
Wynu, Seraph: Spell: Summon Tiny Ring (Level 16)
There are two rewards for wizard. They are two spells of the same line. One for levels 25 and under and one higher than that. Your level will dictate which one you get.
Ruhyk, Bernard: Spell: Lesser Familiar (Wizard 45)
Here is a summary of characters that have the task and where they are in the competition.