There is nothing like ishtadev which is nothing but an invention of the UP pundits sitting on the banks of Ganges. All devas are ishtadevas and there is no such definition for an ishtadev. Gods are all all-powerful and you can treat any one of them as your favourite God. After all Gods are one and the same.
If you are not satisfied with this answer, you can search for IshtaDev on the internet and you will get all rubbish information about it and how to propitiate your ishtadev.
Being from Quilon, you can consider Shaktikulangara Temple God as your ishtadevi.