Back of the phone was the best. I could pull my phone out of my pocket and unlock my phone in single motion and decent grip. Plus the swipe gestures for accessing the notifications bar...
I am rocking a Pixel 4a and it still has the sensor on the back, I couldn't possibly go back to the dodgy sensors on the phones like the Samsung Galaxy S5 or J7 Pro which had them on the skinny oval home button. Likewise I still have an iPhone SE and I hate the Touch ID on it, it's so awkward to use.
The back fingerprint sensor needs to stay, absolutely the most ergonomic way to unlock my phone I've ever had.
Despite enjoying multiple models of the phone I currently have I won't get a new model because the sensor is on the power button. Back of the phone supremacy under screen is OK if it works, power button sensor can burn in a hole.
I have a Pixel 6 with underscreen fingerprint reader and I love it. Only wish it was a tad faster and a little less prone to not recognising my thumb occasionally. Hopefully the tech is already better in newer models.