Besides a CEO, who should a Head of Sales have as their biggest ally at the company?

Easily the head of marketing

Got to be Marketing and the Sales Development

Head of Marketing.  Marketing and Sales need to operate hand in hand.  Sales is the front lines to customers - they learn the language and the objections, the temperature of the market, the selling points.  Marketing can then take these insights and leverage them for messaging and content.  When marketing and sales are at odds, it is an actual nightmare. I have worked in both situations (harmonious and then misaligned) and leaders who are not in tandem between sales and marketing create more problems... and that discord trickles downstream into their respective teams as well.  We are ALLIES not opponents!