Pixel 8 has the best security with hardware Memory Tagging support. A Pixel 7a is the best for value and lifespan. A pixel 6a is the cheapest of all supported devices with more than a year of support left.

Worth noting Google is making an announcement for Pixels in a few days... Pixel 8a?

How long can GrapheneOS support my device for?
https://grapheneos.org/faq#device-lifetime