i offer wonder where the state of the web would be if John Resig never would have created jquery. i think everyone who is into web development today owes a thank you to him. jquery single-handedly made programming on the browser easier.
Besides jQuery there were several competing libraries abstracting the browser inconsistencies: YUI (Y=Yahoo), prototypejs, mootools, dojo. So I'd say multi-handedly.
Strong leadership without the bickering and bikeshedding that ruined its competitors.
The first time around was rails with HAML/sass, with a light sprinkling of jquery used like CSS, mainly to make things like sortable tables, search w/ typeahead find, and a few other fancy things work. The second time around I used rails as a REST api and wrote an angular 1.x SPA for the frontend.
Honestly, the first time around was far, far nicer from a developer ergonomics perspective. Just writing your HTML in nice reusable, composable templates, even though there's a lot of scary abstraction violations being able to access @variables from your ruby controllers, was a very straightforward experience. And the jquery was used mostly like CSS with something to the effect of `$("input.search").search({...});` to just apply a jquery plugin to all css-selected elements indiscriminately. And you know what? It was pretty darned easy! Move onto the next problem.
My angular experience was far more complicated, and building the SPA was a level of effort on par with designing an entire application from scratch, with almost as much time spent developing it as I spent on the backend code.
You can talk a lot about how jquery isn't necessary because you can do equivalent stuff without it, and I'd totally agree, but it was just the perfect tool for what I needed it for at the time. Lots of awesome plugins to enhance (with graceful fallbacks) the views that rails spit out, without me having to write anything. Angular had lots of cool plugins too but the level of effort was just too high.