Assuming, your range is A1:A10, enter the below formula as Array Formula i.e. not by pressing ENTER after entering your formula but by pressing CTRL+SHIFT+ENTER. This will put { } brackets around the formula which you can see in Formula Bar. If you edit again, you will have to do CTRL+SHIFT+ENTER again. Don't put { } manually.
=INDEX(A1:A10,MATCH(MAX(COUNTIF(A1:A10,A1:A10)),COUNTIF(A1:A10,A1:A10),0))
The non-Array version of above formula
=INDEX(A1:A10,MATCH(MAX(INDEX(COUNTIF(A1:A10,A1:A10),,)),INDEX(COUNTIF(A1:A10,A1:A10),,),0))