I few years ago, I was running Houdini with Redshift and loved it.

I would love to hear some suggestions for what render engine I should come back to and why.

I'm a bit of a houdini hobbyist (for now) working in comp, vfx, motion graphics for my day-to-day.

I would love to use houdini more, but I need to get my skills back up.

Thanks for any and all input and information

I've been using Redshift with Houdini for about 4 years, but last year I decided to dive deeper into Karma and honestly, I haven't looked back.

Karma XPU is both GPU&CPU based, but you can only use it with GPU if you prefer. One of the biggest advantages for me has been working with pyro shaders, what you see in viewport is close enough to what you get, making it less time consuming compared to Redshift.

For freelancing & collabration, I've been also using Arnold, but my experience has been hit or miss. It might be just me, but I found myself spending too much time troubleshooting random issues, where with materialX & karma, I was getting expected results straight out of the box.

If you're returning to Houdini, I'd say Karma is definitely worth exploring.

Try Karma first and if you still feel the need for 3rd party rendered, then try Redshift and others.
I am sure Redshift is by far most popular even Karma is getting more and more users just because it is getting better and most cost effective.

I think they probably should have treated Karma as a plugin and develop it like that, iterating more than what you get with 20, 20.5 etc...

What about Renderman? and their non-commercial version (to learn)?