Bastard is a Windows machine with interesting Initial foothold. There is some PHP knowledge needed, although the changes need to be done for the exploit code are pretty minimal. There are more than one way to get into machine!
Popcorn is relatively easy Linux machine although initial foothold is not very straightforward. You need to dig a bit to find it. Because this machine is running on a pretty old Ubuntu version, there are more then one way to escalate privileges.
Devel is a relatively easy hackthebox Windows machine, which can be done almost all the way with metasploit.
October is a relativly easy to moderate Linux machine, that has a fairly nice entry point. What makes this machine very interesting though is the privilege escalation part. It includes buffer overflow with some obstacle(s) on a way = ASLR
Beep is an easy Linux Box with more Services running. Unfortunately the way to the root is very unspectacular and most of the running services don’t really do anything and are plain rabbit holes.