Shankar is called as Anishwar because there can be no one exist as a Lord of him. He is the controller of all, but he is not controlled by anyone. If someone is higher to him, then only his regulation is possible. Brahma and Vishnu were failed to wholly see just his Jyotirlingam. Then how can they be superior to Shankar? They are actually pupils of Shankar. While Brahma and Vishnu are pupils, then who will administer him? Goswami Tulsidas ji has expressed this notion like this— param svatantra na sir par koi. While creator and maintainer are set aside, then what to speak about demigods like Indra, Varun, Agni etc? Shiva is called as Anishwar because he is the supreme controller of all. Shiva only is left behind after total cosmic annihilation and next era is beginned from him only. At that time everything depends upon him only, so Shiva is named as Anishwar.
In the spiritual science, Adhyastha (supported) is always dependent on Adhishthan (Prime support), but prime support depends on nothing. Therefore he is called as Anishwar. The Vedic hymns sing the chorus in praise of his supreme proprietorship— na tasya kashchit patirasti loke na cheshita.
The definition of Anishwar in the words of Acharya Bhaskar is given below—

ईश्वरत्त्वावधिर्यः स सर्वेश्वरः। तस्य नेशः परः त्वं ततोऽनीश्वरः॥

O Lord! Whatever is the summit of Godliness, that supreme Lord is none other than you. No one else may claim to be your controller. Therefore you are Anishwar. God is the only organizer of actions because he sanctions the fruit of actions. Shiva rejects that fruit also, so he is called as Anishwar. Another meaning of Anishwar is one who is not a controller. Shiva is known as Anishwar because he is controlling everything and simultaneously not controlling also. Appreciating the supremacy of Lord Shiva is the true sense of worship of Anishwar.