For biblatex the first choice is most definitely biblatex-examples.bib, which David Carlisle already pointed out in the comments. The file is installed in a place where both BibTeX and Biber can find it and comes bundled with biblatex, so is usable without intervention on any system that runs biblatex (at least if biblatex is installed correctly).
biblatex-examples.bib contains a variety of entries demonstrating a wide range of standard biblatex features. For many common use cases there should be an entry in that file.
I generally try to avoid \nocite{*}ing the whole file since that results in about six pages of bibliography output that probably drowns out the point I am trying to make, so there are several entries like sigfridsson (@article), nussbaum, worman (@book), geer (@thesis), westfahl:space (@incollection) that I turn to quite often and know by heart already.
\documentclass[british]{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{babel}
\usepackage{csquotes}
\usepackage[style=authoryear, backend=biber]{biblatex}
\addbibresource{biblatex-examples.bib}
\begin{document}
Lorem \autocite{sigfridsson}
ipsum \autocite{worman}
dolor \autocite{nussbaum}
sit \autocite{sigfridsson}
amet \autocite{geer}
\printbibliography
\end{document}
Note that most entries in biblatex-examples.bib use date (instead of year even if the date consists only of a year) and journaltitle instead of journal. The file is therefore usually not that great of a resource for BibTeX styles.
For BibTeX there is xampl.bib, which comes with the standard BibTeX installation. I can't quite put my finger on the exact reasons, but somehow many entries from that file feel a bit clunky to me.
apacite comes with a very comprehensive apa5ex.bib.
As Marijn rightfully points out in the comments, sometimes a question really depends on some specific feature of your .bib entries, which pre-made example files might not have. In that case filecontents comes in extremely handy to make your example self-contained. (Recall that the default filecontents environment does not overwrite existing files. In newer versions one uses the optional keyword force or overwrite, i.e. \begin{filecontents}{\jobname.bib}, in older LaTeX versions one would load the filecontents package, i.e. \usepackage{filecontents}, to allow overwriting of existing files.)
\documentclass[british]{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{babel}
\usepackage{csquotes}
\usepackage[style=authoryear, backend=biber]{biblatex}
\begin{filecontents}{\jobname.bib}
@book{appleby,
author = {Humphrey Appleby},
title = {On the Importance of the Civil Service},
date = {1980},
}
\end{filecontents}
\addbibresource{\jobname.bib}
\addbibresource{biblatex-examples.bib}
\begin{document}
\cite{sigfridsson,appleby}
\printbibliography
\end{document}
\addbibresource{biblatex-examples.bib}with biblatex – David Carlisle Jun 16 '20 at 10:38.bibfile which has the problem that a predefined.bibfile may be hard to find and open in an editor. – Marijn Jun 16 '20 at 11:54