Having a license, whatever it is, is much much better than not having any one at all. So, if you can’t decide, put at least something. You can always change the license later (of future versions - you obviously cannot change the past).
Many options depending on your objectives. For scientific work, I often find it useful to put myself in the shoes of people who’ll find my work. They’ll ask themselves, if they find the artifact (code, document, …) useful, questions like:
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Am I allowed to use this in what I’m doing right now?
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If I improve it and contribute back, will my contribution be available to others? Is it worth my time? Is it fair?
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Am I allowed to improve upon it and then redistribute at my own will? (e.g. part of a package, a document, a publication, …)
If I find work of interest without a license, I try to reach out to the authors an convince them to add a license. Working with non-licensed artifacts is quite uncertain. You may think you’ll do something quick, but then you find a bug and you fix it, and then you’re in a limbo whether you can share that fixed version with the world or not.
If one of the copyright holders (typically all authors) passes away, then I don’t even wanna think what it takes to get a license in place. Legally, will that be the end of that artifact? I assume this will be a more common problem as more and more artifacts are produced each day. BTW, Software Freedom Conservancy is one organization that thinks about the copyright beyond the life of software maintainers.
EDIT: Added link to Software Freedom Conservancy.