Small, focused helper functions that can be combined with any number of other functions, in any combination.
The downside to bundling them all together is if you only want to use one part of it, you have to start making decisions about what to do with the code you intend not to use. Tree shaking? Custom build? Import anyway? With ES modules does it make sense to have to download a giant file only to read one function from it? etc.
I think, especially with the nature of JavaScript modules, that single-function helpers are probably the most flexible.
But, you raise a good point as well, sometimes there's a theme that ties a number of helpers together around a common task, a common purpose, or a common functionality. What are ways we can package up and market sets, series, or families of plugins together in a way where they can be used independently from each other too?