I think the most common format is the one that is suitable for the purpose. What your customer are planning to do with the data? Do they want to be able to analyze it and run some reports? Do they want to be able to re-create the database? Do they need an ability to load it into another DBMS? ASCII files (CSV or not) are the lowest common denominator, but they must be accompanied by extensive documentation describing their format, otherwise in 6 months you will struggle to find anyone who has a clue about what's in there. XML is more verbose but is somewhat self-documenting. On the other hand, you might just as well give them a compressed DB2 backup image...