I think you might be bombing you UI with JS by asking such a question. If you want to have minimal UI code, try to get away from JS as much as possible. Try to do stuff with modern HTML and CSS as much as possible.
This being said, there are many good libraries out there which all try to be good at whatever someone created them for. So it really depends on what actions an admin should be able to perform and how you want to present data. For example if you have a lot of widgets which display editible data, you might want to go with some Flux library, like React. If you have a lot of data you just want to display in a very nice way, give d3 a try. If you have a lot of static content and forms, you don't really have to bother with JS (except you want to load content via AJAX of course).
Maybe you could elaborate a bit on what you plan to do?