DNS server software

Hello all, We are a mid-sized carrier (1.2M broadband subscribers) and we are looking for an upgrade in our public DNS resolver infrastructure, so we are interested in getting to know what are you guys using in your networks. Mainly what kind/brand of software and which architecture did you use to deploy it, and how did you do the sizing, all of it would be most helpful information. Many thanks in advance for your advice! cl.