Okay, first off, since you are asking for "best," and I'm quite sure this is not a historical question about the human past, this might very well be against the rules.

But should that not be the case:

There is no single "best." The usefulness of an education in another field of study for history depends on what research interest you have in history - or, more specifically: what research you will do in the future (and who really knows that beyond the next thesis or, later, project?!).

There can be made an argument for multiple disciplines out there; off the top of my head I can immediately think of:

Anthropology. Archeology. Information Sciences (Computers). Linguistics. Literature. Philosophy. Political Science. Psychology. Sociology. (some of these are more obvious than others)

For example, studying computer sciences will open a path into digital humanities, big data, AI-supported quantitative research methods, and so on.

Other disciplines might give you a baseline education in certain theories and concepts which are immediately applicable in historical research. (This also means that at a certain level, you are often expected to know at least the big authors from these other fields which directly pertain your specific topic of research as a historian anyway.)

Take anthropology: this should teach you the fundamentals of kinship theory. This means that you don't need to read up on kinship from scratch when researching family or marriage as historical phenomena. But if you analyze the political machinations during the Cold War period you might profit more from a previous education in political science.

Suffice it to say, for this reason, you can only guess what might be "best" based on your general interests on topics in the very wide field of history, and what might be useful when pursuing this general trajectory.