There cannot be a greatest common factor of just one number. To be common there need to be at least two numbers. If you find all the factors of two or more numbers, and you find some factors are the same ("common"), then the largest of those common factors is the Greatest Common Factor.
The common factors of 45 and 108 are: 1, 3 & 9.
The common factors of 81 and 108 are: 1, 3, 9, and 27
The common factors of 81, 108, and 162 are: 1, 3, 9, and 27
The common factors of 24 and 108 are: 1, 2, 3, 4, 6, and 12
The highest common factor of 108 and 45 is 9
Since 18 is a factor of 108, all of its factors are common, i.e. 1,2,3,6, 9 and 18.
The common factors of 108 and 756 are 1, 2, 3, 4, 6, 9, 12, 18, 27, 36, 54 and 108.
Since 54 is a factor of 108, all of 54's factors are common and the GCF is 54 itself.
Since 36 is a factor of 108, all of its factors are common.
3 is the greatest common factor of 108 and 111.
54
The greatest common factor is 4