Horizontall
A fun box, it took me an embarassing amount of time to get the privesc to root working, despite knowing exactly what I needed to do! A box that rewards good thorough enumeration
HackTheBox.eu
Possibly the most well known gamified hacking site, imho this is not the best place to start for a beginner (although they are making changes which could support beginners more), but it is a fantastic site once you know a few things, with a great range of boxes showcasing some really clever and well designed challenges
Armageddon
A linux box, user rated Easy/Medium, Armageddon is a bit of a black & white situation, if you find the user exploit fast then pretty easy, otherwise you could be looking for a while. Root on the other hand is a pretty simple privesc
Blunder
A linux box, user rated Medium, fairly straightforward apart from bruteforcing protection on the initial access