That depends on your budget, your device, your risk tolerance, and your aesthetic expectations.
For most people, a tempered-glass protector will be the best choice, because it offers the highest level of damage prevention and has a smooth touch. It's more expensive than the other options, and you'll probably notice a slightly altered look to your phone screen, but if damage control is your goal, get a tempered glass protector.
If appearances matter most or if you're in the market for a basic, cheap option for an old phone, the plastic screen protectors (PET and TPU) are sleeker options, though again, they're less scratch resistant.
i agree, i use tempered glass, but cheap one, because my main goal is only to protect my screen from scratch from my thumb. Based on my experience my screen will be easier to scratch if i use gesture navigation, so i prefer using tempered glass and replace it when needed
