So you want to save the world? Get an iPhone SE.
Apple publishes environmental report cards for many of their hardware products on their Environment page. These reports contain detailed information about the environmental impact of production, lifetime use, recycling and more for each device. An estimated life cycle carbon emission value, measured in kilograms of Carbon dioxide equivalent, is provided for each device.
Most of the emissions occur at production of the phones and, as is evident in the chart above, the larger the phone the more carbon emissions are required to manufacture and ship it to the customer. As Apple has gone all in on larger devices their environmental impact has been slowly trending up over time despite the massive effort the company puts into making every step of the process as environmentally friendly as possible.
Of the phones that support the latest iOS version the iPhone SE is the device with the lowest impact at 45 kg CO2e, which is only about half of the impact of the massive iPhone 11 Pro Max. Some years earlier the even smaller, and to my eyes yet more beautiful, iPhone 4 had achieved the same low level of emissions.
There are some glaring omissions in Apple's list of product environmental report cards which is why the chart is missing data for the original iPhone, iPhone 5 and 5c, and iPhone 6 and 6 Plus.