Greatness of Lord Surya is described in Vana Parva in Section 3.
Yudhishthira said, "Thou art, O sun, the eye of the universe. Thou art the soul of all corporeal existences. Thou art the origin of all things. Thou art the embodiment of the acts of all religious men. Thou art the refuge of those versed in the Sankhya philosophy (the mysteries of the soul), and thou art the support of the Yogins. Thou art a door unfastened with bolts. Thou art the refuge of those wishing for emancipation. Thou sustainest and discoverest the world, and sanctifiest and supportest it from pure compassion. Brahmanas versed in the Vedas appearing before thee, adore thee in due time, reciting the hymns from the respective branches (of the Vedas) they refer. Thou art the adored of the Rishis. The Siddhas, and the Charanas and the Gandharvas and the Yakshas, and the Guhyakas, and the Nagas, desirous of obtaining boons follow thy car coursing through the skies. The thirty-three gods 1 with Upendra (Vishnu) and Mahendra, and the order of Vaimanikas 2 have attained success by worshipping thee. By offering thee garlands of the celestial Mandaras 3 the best of the Vidyadharas have obtained all their desires. The Guhyas and the seven orders of the Pitris--both divine and human--have attained superiority by adoring thee alone. The Vasus, the Manilas, and the Rudras, the Sadhyas, the Marichipas, the Valikhilyas, and the Siddhas, have attained pre-eminence by bowing down unto thee. There is nothing that I know in the entire seven worlds, including that of Brahma which is beyond thee.