"The strong is always right." is incorrect.
"The strong one is always right." is correct.
"The stronger is always right." is incorrect.
The first would be correct if you changed 'is' to 'are' as in, "The strong are always right". This is in fact, a fairly common English saying.
The third would also be correct if you change 'stronger' to 'stronger one'
"The stronger one is always right"