This function seems to be missing from the list of functions.
In what piece of the product?
And, why? Excel has it, but you don't need it: do a count and take the TOP1 of the count measure, job done.
If you want to suggest that as an improvement, go to https://community.oracle.com/products/oracleanalytics/categories/idealab-oracle-analytics-cloud-server and post an idea (that's where ideas for new features or improvements are collected).
If you can do it in excel, you should be able to do it in OAC classic analysis. I'll have to put it in the Idea lab.