Utumno, the strongest fortress every built?

[Zigûr]

Melkor had also, seemingly, not previously wasted so much of his native potency as he did in the Battle of the Powers; indeed, 'Myths Transformed' suggests that the very act of empowering his servants in the defence of Utumno (or beforehand, such as in the destruction of the Lamps) was perhaps the first great reduction of his strength, with both Manwë and Melkor himself 'amazed' at this weakening: "he has now less personal force than Manwë, and can no longer daunt him with his gaze."

I think it's all relative to the strength of the defender's opposition, however.