Least used delimiter character in normal text < ASCII 128

Assuming for some embarrassing reason you can't use CSV I'd say go with the data. Take some sample data, and do a simple character count for each value 0-127. Choose one of the ones which doesn't occur. If there is too much choice get a bigger data set. It won't take much time to write, and you'll get the answer best for you.

The answer will be different for different problem domains, so | (pipe) is common in shell scripts, ^ is common in math formulae, and the same is probably true for most other characters.

I personally think I'd go for | (pipe) if given a choice but going with real data is safest.

And whatever you do, make sure you've worked out an escaping scheme!

I would choose "Unit Separator" ASCII code "US": ASCII 31 (0x1F)

In the old, old days, most things were done serially, without random access. This meant that a few control codes were embedded into ASCII.

ASCII 28 (0x1C) File Separator - Used to indicate separation between files on a data input stream.
ASCII 29 (0x1D) Group Separator - Used to indicate separation between tables on a data input stream (called groups back then).
ASCII 30 (0x1E) Record Separator - Used to indicate separation between records within a table (within a group).  These roughly map to a tuple in modern nomenclature.
ASCII 31 (0x1F) Unit Separator - Used to indicate separation between units within a record.  The roughly map to fields in modern nomenclature.

Unit Separator is in ASCII, and there is Unicode support for displaying it (typically a "us" in the same glyph) but many fonts don't display it.

If you must display it, I would recommend displaying it in-application, after it was parsed into fields.

Probably | or ^ or ~ you could also combine two characters

When using different languages, this symbol: ¬

proved to be the best. However I'm still testing.