From the FAQ:
8th/9th generation Pixels provide a minimum guarantee of 7 years of support from launch instead of the previous 5 year minimum guarantee. 8th/9th generation Pixels also bring support for the incredibly powerful hardware memory tagging security feature as part of moving to new ARMv9 CPU cores. GrapheneOS uses hardware memory tagging by default to protect the base OS and known compatible user installed apps against exploitation, with the option to use it for all apps and opt-out on a case-by-case basis for the few incompatible with it.
In addition to important hardware security features and longer support time, the 8+ Pixels also have USB-C to DisplayPort output support, which means it can be plugged into an external monitor. This will be more useful once the desktop mode and virtual machines features are released.
Pixel 9s also have increased hardening of the cellular modem.
In conclusion, if you have the money I'd recommend getting a Pixel 8+, and ideally the 9 series, due to significantly increased security, longer support time, and the DisplayPort feature. If cost is a concern, then Pixel 6s and 7s are still decent, but I'd first look for a deal on the Pixel 8a. And remember to account for the support period when buying a device.