Japt -h, 8 bytes

Input as a multi-line string

·ñÊÔñ!èU

Try it

·ñÊÔñ!èU :Implicit input of multi-line string U
· :Split on newlines
 ñ :Sort by
 Ê : Length
 Ô :Reverse
 ñ :Sort by
 !èU : Count occurrences in U
 :Implicit output of last element

Japt -g , 9 bytes

Input as an array of lines.

ü üÊÌcÌñÊ

Try it

ü üÊÌcÌñÊ :Implicit input of array
ü :Group & sort by value
 ü :Group & sort by
 Ê : Length
 Ì :Last element
 c :Flat map
 Ì : Last element
 ñ :Sort by
 Ê : Length
 :Implicit output of first element

Japt -g, 9 bytes

Input as an array of lines.

ü üÊÌcÌñÊ

Try it

ü üÊÌcÌñÊ :Implicit input of array
ü :Group & sort by value
 ü :Group & sort by
 Ê : Length
 Ì :Last element
 c :Flat map
 Ì : Last element
 ñ :Sort by
 Ê : Length
 :Implicit output of first element

Japt -h, 8 bytes

Input as a multi-line string

·ñÊÔñ!èU

Try it

Japt -g, 9 bytes

Input as an array of lines.

ü üÊÌcÌñÊ

Try it

ü üÊÌcÌñÊ :Implicit input of array
ü :Group & sort by value
 ü :Group & sort by
 Ê : Length
 Ì :Last element
 c :Flat map
 Ì : Last element
 ñ :Sort by
 Ê : Length
 :Implicit output of first element

Japt -h, 8 bytes

Input as a multi-line string

·ñÊÔñ!èU

Try it

·ñÊÔñ!èU :Implicit input of multi-line string U
· :Split on newlines
 ñ :Sort by
 Ê : Length
 Ô :Reverse
 ñ :Sort by
 !èU : Count occurrences in U
 :Implicit output of last element

Japt -g, 9 bytes

Input as an array of lines.

ü üÊÌcÌñÊ

Try it

ü üÊÌcÌñÊ :Implicit input of array
ü :Group & sort by value
 ü :Group & sort by
 Ê : Length
 Ì :Last element
 c :Flat map
 Ì : Last element
 ñ :Sort by
 Ê : Length
 :Implicit output of first element

Japt -g, 9 bytes

ü üÊÌcÌñÊ

Try it

ü üÊÌcÌñÊ :Implicit input of array
ü :Group & sort by value
 ü :Group & sort by
 Ê : Length
 Ì :Last element
 c :Flat map
 Ì : Last element
 ñ :Sort by
 Ê : Length
 :Implicit output of first element

Japt -h, 8 bytes

Input as a multi-line string

·ñÊÔñ!èU

Try it

Japt -g , 9 bytes

Input as an array of lines.

ü üÊÌcÌñÊ

Try it

ü üÊÌcÌñÊ :Implicit input of array
ü :Group & sort by value
 ü :Group & sort by
 Ê : Length
 Ì :Last element
 c :Flat map
 Ì : Last element
 ñ :Sort by
 Ê : Length
 :Implicit output of first element