What is the most “simple” Photo Gallery plugin?

I use Gallery 2. It’s not so much a plugin as a stand alone that is very developed.
For your plugin, I would “assume” you go to
Admin -> Options -> FAalbum
or maybe
Admin -> Manage ->

I use FAlbum and once it’s set up it’s probably my favourite gallery plugin. Slightly annoying to customise if you plan to update it in the future, but good anyway.

Once you’ve activated the plugin the options are available at Options – FAlbum. Set up the Flickr authorisation, then if you haven’t turned on friendly URLs you can view the gallery at https://yoursite.com/wp-content/plugins/falbum/falbum-wp.php, or if they’re turned on the default will be https://yoursite.com/photos/. It might need some tweaking depending on your theme, but basically it should just be taking your photos from Flickr and displaying them in a gallery at this point.

Give this a try if you have time:

https://plugins.atterberry.net/lazy-k-gallery

It’s very simple to use and it works very well.