Not such a simple thing as it seems.
There are several parts to this, adding in the free thing cuts your options a bit. Firstly it will depend on how many photos you want to store and how big they are. High res photos will eat your free storage quotas up quickly but if that's not a priority then something like Google photos or Dropbox will most likely be fine.
Security is another problematic one, if you mostly mean keeping your photos private and/or being able to share only specific ones with specific people then both Google and Dropbox give you lots of control that way. Of course anything Google is subject to them using their various AI based scans which sort and categorize the images. No humans see them but Google does harvest information from them.
There are a plethora of other, less mainstream options as well, however if you really want to ensure long term access choosing a large provider is really the safest option as these smaller places are prone to simply disappear sometimes.