Atlas 4mm Z-Plug
The Z plug has with some justification, become the plug of choice for speaker cables. The most popular and arguably the best of these is the solderless crimp plug. The conductor to plug interface can be crimped under pressure to amalgamate the plug metal with the conductor metal to form an air tight seal that will not oxidize with time. It's also available for connection to the conductors via grub screws where crimping is not an option.
The 4mm contact area is produced from beryllium copper; it expands within the socket to make contact around 360 degrees and that makes a better signal path than the typical banana plug with a spring for example that only has contact via the spring and the part of the periphery of the plug opposite the spring.