- 2008 to 2012: Jquery
- 2012 to 2014: Backbone
- 2014 to 2016: Angular
- 2016 to present: React
And although there are serious contenders, React still seems like it has a solid lead over all others. If it lasts another couple years, it's been the dominant tool for longer than most.
* Based entirely on "what I was hearing the most about at that time," not on any rigorous methodology. There was still plenty of Jquery code being written last year and there are large, mature codebases out there in Backbone and Angular.