People here are saying Oneplus, but I would recommend Pixel 4/5 or Xiaomi Mi A3 with stock Android One to fallback on if you don't like using LineageOS.

Oneplus have become quite end-user hostile lately. They used to be quite good for this stuff but times do change.

If you go the Pixel route, there's also GrapheneOS (formerly Copperhead). It offers a lot more security features, with of course some tradeoffs when you make a phone secure.

https://grapheneos.org/

Most Motorola phones have good support for custom ROMs, although you should of course check your specific model against lineage's list or XDA before buying it. In particular, Motorola doesn't do anything to prevent you unlocking the bootloader; you just get an unlock request code from fastboot, plug it in to their website to get an unlock code, and then give that back to the phone over fastboot and your bootloader is unlocked. They also have SD cards and 3.5mm audio jacks on most (all?) models, if you care about that. The only downside that I recall off the top of my head is that their NFC support is really spotty; a few models have it, but most don't.

OnePlus devices have been always recommended on their Reddit, but personally I've been using a Xiaomi Mi 8 for nearly 2 years with no issues.

Definitely Fairphone: https://www.fairphone.com/en

I just settled on a pixel 3 xl. It has still one of the best camera quality available. Got it on ebay for 135€. OS Lineageos microg ROM. But kept stock camera app.

I was not happy with the selfie quality on the pixel 3a xl although I would have liked the audio jack.

I was hoping to port it to the LG Velvet, especially since LG is debating leaving the cell phone space

Unfortunately they geolock unlocking their devices. European models can be unlocked, Australian ones? Nope!

Anything from OnePlus, they're consistently friendly with the custom rom scene.