One of A.ROOT-SERVERS.NET through G.ROOT-SERVERS.NET (and perhaps more)
These systems are the backbone of DNS resolution. All unresolved DNS queries are forwarded to one of these hosts. These systems contain information on authoratative systems for all the first level domains such as .com, .net or if you're outside the USA, .au, .uk, .cx and so on.
For any DNS server to operate correctly it must first be loaded with the IP addresses of these hosts - since by definition "A.ROOT-SERVERS.NET" isn't particularly helpful when you can't resolve a hostname!