Raistlin Majere hands down. Has an amazing journey, life story, and the most power.
Traveled through time to murder the wizard whom he owed his physical body, robbed him of his life force, challenged the goddess of evil to eliminate her and take her place, destroyed a pantheon of gods and an entire universe, but ended up being influenced by his twin brother at an earlier date and purposefully losing his fight with the goddess in order to save the world before he became too powerful/evil to stop himself.
The bond between him and his brother was heart wrenching as hell too.
Even before he reached his peak his power was more than any wizard I've ever read about. The dude would cause earthquakes, lightning storms, command ancient dragons, summon twisted demons too insane for the average human mind to even process, and he actually decimated an entire mountain range with a spell gone wrong, yet held his body together through sheer will. A fucking death knight bowed to him and didn't dare lift a finger against him. A death knight -- a being powerful enough to point at any living thing and say "Die!" and kill it.
He was also incredibly interesting because he was so ambitious and ended up doing some truly evil things, but every now and then he shows enough compassion to make people love him, like sacrificing himself and permitting eternal torment at the hands of the dark queen in order to save the world he knew he would destroy later on if he beat the dark queen.