My most used tool, by far, is the debugger, in any language. Imho, it's the most essential tool to take a peek into a running application to identify logical errors. For example:
By your list of tools, though, I think you mean inside the browser dev tools, right? In this case, in addition to the debugger, it's probably the networking tools and storage overview. The networking tab gives me vital information about what was loaded, if it is cached, how long loading took, what frames a websocket sent, etc. The application tab gives me insight into the data my application stores inside the browser, which is important. It allows me to view, modify and delete the data and test stuff this way.
I arranged my tabs like this: