Aigbids Depends on your criteria:
- Security-wise, the 9th series is the best as it has the most-recent radio. Then, the 8th series devices are also a step up above all the earlier ones (ARMv9 with memory tagging).
- Battery longevity was supposedly improved starting from Pixel 8a (so not including the other 8th series models): it should retain 80% capacity after 1,000 cycles, compared to 800 cycles before.
- In terms of affordability, the 6a is unbeatable as you get practically all the same features, including great camera, and it's the smallest device you can run GrapheneOS on (although it only has 2 more remaining years of active support and there is battery-related uncertainty).
- In terms of photo and video quality, there hasn't really been an improvement since the 8 Pro: however, the 9 Pro delivers pretty much the same in a smaller form factor, and also improves the fingerprint reader and the radio.
You could invert the question and think which choices are strictly dominated by others and thus not worth considering:
- At this point I'd skip the 6th and 7th series entirely, except for the 6a, for reasons mentioned above.
- I'd then skip the 8a and the 9a since they're significant downgrades from each of the respective non-a models yet their prices are only marginally lower (around $50 difference currently I think).
- I'd skip the non-Pro 9 since it's a huge downgrade from the 9 Pro yet they're the same in terms of size, so by giving up all the extra features you gain nothing in terms of portability.
- 9 Pro XL only offers marginal improvements over the predecessor: if you can accept their gargantuan dimensions, you might as well go for the 8 Pro if you can find it for much less.
Of course this all depends on the market situation as well: at certain price points, any of the Pixels might be worth considering. But I'd probably be picking from among the 6a, the 8, the 8 Pro, and the 9 Pro. It's of course just my subjective opinion but I've outlined my criteria.
In any case, I wouldn't buy one now as the release of the 10th series is just around the corner. Even if the 10th series ends up being a disappointment and/or lacks GrapheneOS support at least initially, it should at the very least drive down the prices for the earlier devices.