Most powerful was a while ago, D'Gatham of Cormyr. Or, as he was known later in his adventuring career Songmaster D'Gatham, Patriarch of Milil.
These days I play an evening every other week, back in high school and college we'd regularly do weekly 12-14 hour marathons. Back in AD&D 2nd times I ran one character for close to a decade of that intense schedule. He started as a bard, and at 7th level dual classed over to cleric of Milil (god of song) as he tok his postponed vows. Back then demihumans could multiclass, which was pick several classes and advance in all of them, wth your XP slip evenly between the classes (each class had it's own XP chart and leveled up at different points), while humans could not multiclass but could give up a class, take another, and once they bypassed the level of their original class they could use it's features again. Technically they could use it's featues at any point, but if you "fell back on old ways" you didn't get any XP for the adventure.
Anyway, between years of play and also a number of magical manuals that boosted bard level, I ended up an epic (above 20th) level cleric and 14th (?) level bard.
When 4e came out, a friend and fellow player ran another game, though set hundreds of years later since the FR timeline had advanced, and used him as an NPC. D'Gatham ended up being still alive, and basically a spokespiece for the gods of good. FR back then had lots of chosen like Elminster and the Seven Sisters and such, this was him just elevating my character to the same level as NPCs from the books.
