Best way to store files on Arduino
If you want files, then you need a filesystem. The only simple to use and reliable things on an Arduino that have a filesystem are SD cards or USB memory sticks through the USB Host Shield.

However, most of what people think they want to store in "text files" is actually data that can better be stored in other ways. Using text to store numeric values that only the Arduino will ever see is pointless - store it as numeric values.

The most convenient place to store such values (if you don't have lots of them) is in the internal EEPROM.

If you have more than a few values you can use an external EEPROM or serial Flash chip to store larger amounts of data.

Anything more than that, though, and SD card rapidly becomes the most cost effective method.