What is the best file format to work with?

Many users have this question when they start using Transifex.
Transifex supports different type of file formats as described here.

We got this result in our usage research:
1- Key-Value JSON
2- PO file
3- Android (XML)
Strings, YML, HTML, and XLIFF are also quite popular!

What have you been using most and what is the advantages and limitations you see?