OSPF is the best choice for a complex, multiprotocol enterprise network due to its fast convergence, scalability, and support for Variable Length Subnet Mask (VLSM) and route summarization. OSPF is a link-state routing protocol, which means it has a complete view of the network topology, allowing for efficient route calculation and quick convergence. RIP, being a distance-vector protocol, has slower convergence and does not support VLSM or route summarization as efficiently. EIGRP, though it is fast and supports VLSM, is not as widely adopted in a multiprotocol environment as OSPF.