Best format to export data?

I have a lot of data from my website that I want to purge after storing it for a while but I want to keep the information I have so that I can refer back to the stats of my blog posts. What is the best format/easiest way to export the data so that I can review my old data and purge the current statistics to clear up space? CSV? XML? Or TSV? I’m looking for the simplest way to process the file into something that is consumable after exporting, i.e. excel file or just a table.

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