There are many persons who claim to be Bhagavān, but according to the Bhagavad-gītā, Kṛṣna is Bhagavān because He possesses all power, strength, fame, beauty, knowledge and detachment. You can read more in Bhagavad-gītā Chapter 2 – Sāṅkhya Yoga (The Yoga of Analysis) – Verse 8 and the Anuvṛtti