Highest heaven - The five Vimānas (habitats) of the Vaimānika Devas (Empyrean gods)- Vijaya, Vaijayanta, Jayanta, Aparājita and Sarvārthasiddha, which are located above all other heavens.
anuttarāḥ pañcetyādi. vimānaviśeṣāḥ pañca sarvoparyunuttarāḥ avidyamānamuttaramanyad vimānādi yeṣāṃ te'nuttarāḥ devanāmāna eva te vimānaviśeṣāḥ.
(TaBhā 4.20 Vṛ)