What would be the best bus/protocol to use for this???

I'm starting a project where I might have 20-40 devices, each of which can transmit/receive to one another at any time. I'd prefer to use a serial protocol with as few lines as possible. I need to handle device contention but message delivery verification isn't really required. My devices will be chained together within 20 feet of each other. What do you think? What would be the best solution? Since I'm programming all of the devices myself, I could make my own protocol if necessary.