It has to be a pixel to run the official distribution of GrapheneOS as of today.

The Pixel 6a is by far the best bang for your buck. Amazing phone and very cheap. Maybe the 7a is also cheap as well.

Anything over the 6a will be better and more expensive, but less cost effective.

Have a look at the official site: https://grapheneos.org/faq#recommended-devices

They are actually really good at keeping documentation updated.

And the forums are also great: https://discuss.grapheneos.org/