What's the best Anti glare protector for an I pad

Ironically, you’ll get the best glare reduction simply by leaving the screen “naked”. Unless you have one of the cheaper or much older models, iPad screens have a matt anti-glare finish.

Many screen protectors, in particular glass, inevitably increase the screen reflection.

The basic physics are such that each surface transition increases both internal and external reflection. With addition of additional glass (or plastic) layers, you trade-off optical clarity for increased protection coupled with increased reflection and glare.

iPad doesn’t work well at the best of times in bright sunlight; ultimate screen brightness is already limited. Privacy screens use a polariser - which substantially reduces screen brightness; reduced brightness, coupled with increased reflection, makes use of iPad in sunlight even less practical.

There are two further considerations...

First, increased backlight brightness has a significant negative impact on battery life. An iPad operating at high screen brightness will also run much warmer.

Second, use of the iPad in direct sunlight will externally heat the iPad - which is already operating at a higher internal temperature due to high current consumption because of higher screen brightness. Li-ion batteries need to be kept at temperatures the we, ourselves, find to be comfortable. Elevated battery temperatures can cause irreversible damage to a Li-ion battery.

I hope this information and insight proves to be helpful - if only in improving your understanding of the optical properties of the various types of screen protection.

You are very welcome. I hope the information was helpful.