Google Sheets is where everyone starts and is the platform that Appsheet works best with. If your data gets huge (tens to hunderds of thousands of rows) then you might have to migrate to SQL, but the cost increase is significant
I’ve never used GSheets but I heard that it’s considered the best non payed option, although that may be biased coming from Google, I don’t know if anyone have tested it.
I use Excel on all of my projects and it works well in case you wonder