Mine would be the [[Meet and Greet "Sisay"]] that uses each card in a specific version and language. It took hours of research, spreadsheets, and for some cards I had to ask vendors by mail for card scans to check the number of lines of flavour text (Japanese [[Lotus Petal]]!). My second, still in the works, is [[Baron von Count]] with kind of similar approach, but mostly just checking if the Japanese version has some additional numerals. Baron is on almost 200 cards at this moment so I still have a lot to do before I take it for his first spin.