Leaflet is designed with simplicity, performance and usability in mind. It works efficiently across all major desktop and mobile platforms, can be extended with lots of plugins, has a beautiful, easy to use and well-documented API and a simple, readable source code that is a joy to contribute to.
Leaflet uses the OpenStreetMaps database which is very good. It is lacking some of the integrations that Google Maps has, but also some of the horrible spyware.
A classic browser interactive mapping provider. The usual caveats about google apply. For our purposes the important thing is that we can create a Data layer which plots for quantitative data visualisation.
Plotly is not a mapping library but it will plot maps happily as part of other plots.
e.g. Kerry Halupka notes How to create maps in Plotly with non-US locations.
See spatial data in R.
Mapping non-geospatial things
Using maps as visualisations of non geospatial things is a whole other field. What if you can leverage spatial intuition by pretending something is spatial? e.g., as mentioned at knowledge topology, Franka Miriam Brückler’s famous map of Middle Math.
That sounds fun and I'm sure it is studied elsewhere.