Apple

They all take away from the display. Even if nothing else, the simple fact you have one more layer is adding a surface for light to bounce off. Glass is not transparent. Something like 16% of light bounces off most glass, depending on the chemical makeup and also how the layer is bonded to the display (there are ways to bond it that reduce the issue and Apple uses those for all the internal layers on the display).

Having said that — I still think screen protectors are worth it, because i use my phone for a long time and am pretty rough with it, so my glass is going to get scratched and the ability to replace the outer layer periodically (and cheaply) is worth the tradeoff to me.

The best ones don't cover the screen - they cover pretty much the entire front of the device. Unfortunately with some iPhones those are not available (e.g. if the phone has curved edges). And they have an oleophobic coating — because holy shit fingerprints are bad without that. This is another reason to use a screen protector. That coating wears off over time and eventually your factory iPhone screen protector will be covered in smudges (unless you buy a new one regularly).

You also need one with very good installation instructions and maybe also some kind of alignment jig built into the packaging. Wether or not that's necessary depends on what model phone you have, since some iPhones are more difficult than others.

I'm a big fan of D-Brand screen protectors — they're well made and the installation instructions are the best. Even if you buy someone else's protector it's worth watching the D-Brand instructions for tips. This one doesn't have an alignment jig, because one isn't really needed. For some phones or protectors it really is needed. https://youtu.be/OJW89JK3zZk