Best GIS software to work with?

Hi /r/MachineLearning, I was wondering what sort of geographic information systems you all use to generate feature data such as distance to certain points, address intersection with certain municipal areas, etc. I presently use ArcGIS, but the GUI is painfully slow when you have a sufficient number of observations (millions). I'm aware that there are R packages that can do GIS, but I wanted to know what the pros use before figuring out more R.