Which transmission media are the most secure?

Let's start by looking at three broad categories of media: Wire, optic, and wireless. At a high level, optic seems to be the most secure as it is most difficult to tap. Wire is somewhat less secure as it can be tapped without breaking the connection, and can also be monitored without a physical connection (with some effort). Wireless is the least secure since whether it is point-to-point or omni-directional, the signal is easily accessible to many people. Again, this is a high level assessment. There are factors that can increase or reduce the security of each medium, many of those stemming from the implementation of the technology. Some examples:

An Ethernet network may be easily sniffed without detection and be less secure than a securely set up WLAN network. The work effort to break the encryption on the WLAN may be greater than plugging in and sniffing the Ethernet segment. This is a somewhat simple example, but it should show you something. The most secure medium with poor controls and loose implementation may not result in a higher security!

I suggest looking at the situation a bit differently. What are the requirements, both technical and security? Which media can meet those requirements (portability may require wireless)? Can the security requirements be met with proper controls or technology? Then the design of the solution can be developed.