Personally I love the memory foam, harder pillow from IKEA I got for £15. It's so comfortable that other pillows feel like shit compared to it. It's not for everyone though
I went and tried the Purple Harmony pillow when there was a similar thread on Reddit ~1 year ago. It was fantastic! I wanted it sooo bad, but it was just too expensive. I did find out later that Purple runs sales a few times a year. Think the best was around Christmas and it was damn near 50% off. So, I've had it since Christmas and been very happy.
I've got a purple harmony pillow (medium size) and it's okay. I feel like it's too high for back sleeping and not high enough for side sleeping. And due to the nature of it it's nearly impossible to try to mold to the shape I need. I'm thinking of getting a Coop tbh
Use https://www.sleeplikethedead.com/ to look up what works best for you. I went with latex pillows ten years ago and they're still going strong, though starting to smell like latex recently. Probably time to replace them ...
I'm on the buckwheat train. Tried different pillows and ended up with buckwheat. The pillow allows you to mold the shape to an extent which makes it extremely flexible. They also come in different sizes and good pillow sellers will either overstuff them or give you a bag with extra stuffing. You can open the pillow up (via zipper usually) to remove and add more buckwheat for even more customization. It's just extremely flexible overall.
Jumping on the buckwheat train. I was terrible at pushing all my pillows into the bedhead and waking up with a bent neck before picking up one of these. Another benefit is they don't get uncomfortably warm. It's really nice that I can wriggle to flatten it out for back sleeping, or fluff it up to the perfect height for side sleeping.