Within Hinduism, an Ishta-deva or Ishta devata , literally "cherished divinity" from ia "desired, liked, cherished" and devat "godhead, divinity, tutelary deity" or deva "deity") is a term denoting a worshipper's favourite deity.
It is especially significant to both the Smarta and Bhakti schools wherein practitioners choose to worship the form of God which inspires them the most. Within Smartism, one of five chief deities are selected. Even in denominations that focus on a singular concept of God, such as Vaishnavism, the ishta deva concept exists. For example, in Vaishnavism, special focus is given to a particular form of Vishnu or one of his Avatars (i.e. Krishna or Rama), and similarly within Shaktism, focus is given to a particular form of the Goddess such as Parvati or Lakshmi. The Swaminarayan sect of Vaishnavism has a similar concept, but notably differ from practically all Vaishnavite schools in holding that Vishnu and Shiva are different aspects of the same God

Hinduism has about 36b crore devi devtas and the ishta deva is worshipper's favourite deity

like i may worship Hanuman and you might be worshipping Lord shiva