Applied Houdini
Steven Knipping's Applied Houdini is filled with awesome in depth tutorials and provides detailed explanations where needed. He gives the basics, even for free. You should definately check him out.
CGWiki
Matt Estela's CGWiki is one of the first sites I would visit if I encounter some obstacles working with Houdini. But you will not only find Houdini related stuff, Matt has gathered a huge collection of really neat tipps and tricks for other software programs too. Cinema 4D, Maya and Unreal to name a few. Check him out, he deserves some love...
Entagma
Entagma is one of the bigger resources for Houdini tutorials, and the majority is for free. Moritz and Manuel have a really huge compilation of different topics at a very high level in terms of teaching quality. Having it always in my bookmarks.
Junichiro Horikawa
Junichiro Horikawa is a rare item out there. He has so many free tutorials on his YouTube channel that go really deep into Houdini. Most of his tutorials are heavily vex based, what allows him to be as flexible as possible, providing unique topics, not only for Houdini users.
Simon Holmedal
My personal biggest inspiration and the first of many horsemen of Houdini named Simon. His work got me initially started with Houdini. Although he has not really made tutorials yet, his work alone is inspiring and worth watching. Check out his site.
Simon Verstraete
Simon Verstraete is the one which combines Houdini and Unreal Engine. His Artstation is packed up with very cool tutorials about assets and generators. Especially the huge series about his Titan Project is worth watching it.