The 6 is the oldest, but the 6a is cheaper than the 6 and is supported longer as it is their budget version released later. Once a device is no longer supported by Google it becomes unsupported by us.

The 8a is the best overall value as you get 7 years of updates rather than 5 and ALL security features are available. Memory tagging (our top class security feature) isn't available on Pixels earlier than 8 due to hardware. The 6a is the cheapest Pixel overall though.