Which transmission media are the most secure and the least secure?
There are three broad categories of media: Wire, fiber and wireless. On a very high level, it could be said that fiber is the most secure as it is the hardest to tap. Cable is a little more secure, as physical access is not hard to tap and sniff the passing traffic.
Then there is wireless, it broadcasts point-to-point or well beyond the facility in all possible directions. Anyone that can pick up the signal may be able to sniff sensitive information. Although there is a second item we must consider, and that is the physical and technical controls that have been implemented. Wireless can be made more secure by using WPA, encryption or 802.1x.
A cabled network can be fully switched, use encryption and have implemented VLANS. So the point would be that even seemingly weaker systems can have controls implemented to make their security more robust.