Is equanimity (upekkhā) the highest perfection (pāramī)?

The Ten perfections (pāramī) are 

• dāna pāramī (perfection of generosity), 
• sīla pāramī (perfection of moral discipline), 
• nekkhamma pāramī (perfection of renunciation), 
• paññā pāramī (perfection of wisdom), 
• viriya pāramī (perfection of energy), 
• khanti pāramī (perfection of patience), 
• saccā pāramī (perfection of truthfulness), 
• adhiṭṭhāna pāramī (perfection of determination), 
• mettā pāramī (perfection of loving-kindness), and 
• upekkhā pāramī (perfection of equanimity).  

Each of the perfection are practiced simultaneously and one is not more or less important than the other.  It is not possible to practice one pāramī without practicing the other nine.

Please keep in mind that the teachings of the Buddha was first memorized before they were written down hundreds of years after the parinibbāna (physical death of the Buddha).  The teachings were organized in order to aid the memorization and lists were established.  In many instances the lists of items of a teaching are not meant to convey that the items are practiced one after the other.  By investigating the list of items you will discover that it is impossible to practice one item without also practicing the other items as well.