"But his cunning overreached his aim; his words touched too deep, and awoke a fire more fierce than he designed; and Fëanor looked upon Melkor with eyes that burned through his fair semblance and pierced the cloaks of his mind, perceiving there his fierce lust for the Silmarils. Then hate overcame Fëanor's fear, and he cursed Melkor and bade him be gone, saying: 'Get thee gone from my gate, thou jail-crow of Mandos!' And he shut the doors of his house in the face of the mightiest of all the dwellers in Eä."
There are more of course, though most from the Silmarillion call him the most powerful Ainur rather than being.
From Morgoth's Ring:
"Melkor must be made far more powerful in original nature (cf. 'Finrod and Andreth'😉. The greatest power under Eru (sc. the greatest created power).(1) (He was to make I devise I begin; Manwe (a little less great) was to improve, carry out, complete.) Later, he must not be able to be controlled or 'chained' by all the Valar combined. Note that in the early age of Arda he was alone able to drive the Valar out of Middle-earth into retreat."