Metamorphosis
Part of Incognito CTF. A clever box, rated medium, with a simple but effective route to root. Probably about the right level
TryHackMe.com
For me this is hands down the best site for learning everything from the basics up to very advanced challenges. With walkthrough rooms to introduce topics with lots of context and examples up to very hard boot2root boxes where you’re on your own.
A very active and helpful discord is there for help, encouragement and some banter. THM makes it easy to make progress and most of the content is free! (>80%)
RazorBlack
A windows machine, rated medium, this box is pretty hard at times if your Windows skills are not fantastic (like me!). A lot of fun though and excellently made
Year of the Jellyfish
Another box in the ‘New Year’ series by MuirlandOracle. This box is rated hard, but to my mind is not as difficult as some of the other machines in the series
Year of the Owl
A room by MuirlandOracle, rated hard, with a very tricky foothold but then pretty straightforward for a Windows machine
Year of the Fox
A ‘New Year’ series room by MuirlandOracle, rated hard, complete with many rabbitholes and tricks. This one took a while!