Who has been the most powerful villain/villainous team that the avengers have fought and won against??
Korvac, Thanos, Ultron, Kang, Skrull Empire
What story arc is he in?
Am aware he is an Avengers-originated nemesis in the seventies, but mostly heard of him from that one arc in THUNDERBOLTS, where he completely CRUSHED a Thunderbolts-stand-in team known as "the Redeemers".
I think I shall add Count Nefaria, during those occassion, or two, when he absorbed some powers of Living Laser, Power Man / Goliath / Atlas, and Whirlwind, to become a "Super Count Nefaria".