Is there a standard format for sharing data? We are writing a paper, and want to share the raw data.
My advisor suggests that I share the data as an Excel file, but I believe that a plain text file (such as a text-delimited or comma-delimited file) would be better.


Is there a standard format for sharing data?
How do I convince my advisor that a plain text file is a better format for sharing data?
No. 

Depends greatly on the field and kind of data. 

Some fields do have relatively standard formats: 


In Astronomy, FITS is considered pretty much the standard for imagery.
In machine learning, CSV/TSV is relatively common -- though as the data set sizes get larger and richer, other formats come into play. You can troll the UCI ML Repository to get a feeling for what's common.


In general though, I'd argue that plain-text based formats are what you want to preserve. If, happiness of happiness, your paper becomes hugely influential and you only provide an .xls file, you can bet the first thing that's going to happen is a bunch of ersatz plain-text copies will appear. 

If you want to annotate your data, and that's what is swaying your advisor to Excel, perhaps consider something like XML or JSON -- easy for the computer to process and plain text based, yet free-form enough that you can annotate it however you'd like.