Hi

If you are legally separated and your daughter is still your dependent you should be filing as Head of Household.  She would still be your qualifying child until the age of 24.

 If you are separated but have not obtained a final decree of divorce or legal separation by December 31 of a tax year, you can only file as Married Filing Jointly or Married Filing Separately since you are considered married for the entire year. These 2 filing options apply if there are no dependents.

If you were still legally married as of December 31, and can claim a dependent, you may be eligible for Head of Household filing status.  The standard deduction is higher for Head of Household filing status. The tax rates for Head of Household filing status are generally lower than married filing separately. 

Head of household filing status is available  if all 5 of these conditions apply and :

1. You won't be filing jointly with your spouse; and
2. Your spouse didn't live in your home after June (temporary absences due to illness, school, vacation, business, or military service don't count); and
3. Your home was your child's, stepchild's, or foster child's main home for more than half the year; and
4. You paid more than half the costs of keeping up your home during the tax year; and
5. You meet the qualifications to claim the child as your dependent, even if the other (noncustodial) parent is actually claiming the child as a dependent on their return.