The best replacement for xlsx is xls.

The best alternatives to xlsx are rated as following:
★★★★★  xls
★★★  xlsm
★★  xlsb
★  xltx
★  xlconnect
★  xls file
★  xls format
★  csv
  docx

Your alternative for xlsx is not listed? Suggest!
Speaking about xlsx, one probably shouldn't forget about excel 2000.

Other search terms related to xlsx and their ratings are:
★★★★★ excel 2000
★★★ poi
You have in mind a companion for xlsx and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of xlsx in all years. Click on graph for more.
xlsx
xls
xlsm