I tried to put emphasis on the personal nature of the question in the title. I'm not asking for myself or the average individual. I also mean ideal in the way where cost is still a factor. The iPhone 16 Pro has a 1TB model but it's around $500 more expensive than the 128GB version.

I imagine answers are going to vary significantly depending on an individuals approach like relying on cloud storage, SD cards, or a Magsafe NVMe drive for example.

I found 1TB (512GB on the phone and on the SD card) was ideal for me. I could keep the things I wanted for "just in case scenarios" like the files needed for the source ports of Diablo, Half Life, and Morrowind in case I want to play a game or some ebooks if I have time to read. I never needed to uninstall applications and shuffle things. I even had plenty of breathing room.

Another question I'd be interested in hearing people's answers to is what is the minimum storage capacity you would consider for a phone. I don't think I would buy a device with less than 256GB of potential storage. If it has an SD card slot that opens up the potential a significantly.