Integrated SQL queries
The ability to use QUERY, SORT and FILTER without a lot of fooling around, and as arguments in formulas, in the mobile as well as the browser app gives Google Sheets a conclusive advantage over Excel in data analysis.
Helman Frow commented
Agreed. I moved to Google Sheets long ago for basically everything. Occasionally I need to use Excel for something and I'm almost always stymied when I can't use QUERY. It's probably the single best feature in Google Sheets. Also Google Sheets' implementation of FILTER is a lot more useful. Oh, also the REGEX__ functions. Honestly, using Excel now that I'm used to Google Sheets feels like I've time-warped into prehistory.