In terms of raw power, a semi-official monster called "Don Thousand/Monster C" wields a whopping 100,000 ATK (and DEF). It's semi-official because it was designed for and only allowed in unique boss duels where three players cooperate to tackle an overpowered deck.
Then there's the illegal version of Obelisk the Tormentor, wielding an ATK of, well, infinity. However, it's official rendition is reduced to a still formidable 4000 ATK.
For official cards, factoring in effects, I'd pick either Apoqliphort Towers (who was removed from the banned list) or Chaos Ancient Gear Giant, both of whom are immune to spells/traps and offer many additional benefits. Just note that their power comes at the price of difficult summoning conditions and specific archetypes.