=iferror(query('Form Responses 1'!$A:$F, "select C where B='" & $A3 & "' and C<>'' order by A desc limit 1 label C ''", 1))

+--------------------+---------+--------------+-------------+--------------------+----------------------+
|     Timestamp      | Client  |   Contact    |   Address   | Do you like pizza? | Party Date Preferred |
+--------------------+---------+--------------+-------------+--------------------+----------------------+
| 9/20/2017 13:06:36 | Joe     | 555-555-5555 | 123 Main st | Yes                | 9/30/2017            |
| 9/20/2017 13:07:00 | Bob     | 555-111-1111 |             | No                 | 10/10/2017           |
| 9/20/2017 13:07:17 | Joe     |              |             | No                 | 10/27/2017           |
| 9/20/2017 13:07:43 | Jessica | 555-222-2222 | 700 Main St | Maybe              |                      |
| 9/20/2017 13:07:51 | Jessica |              |             |                    | 9/29/2017            |
| 9/20/2017 13:08:50 | Jessica | 555-123-1234 |             |                    |                      |
+--------------------+---------+--------------+-------------+--------------------+----------------------+

+---------+--------------+-------------+--------------------+----------------------+
|         |   Contact    |   Address   | Do you like pizza? | Party Date Preferred |
+---------+--------------+-------------+--------------------+----------------------+
| Bob     | 555-111-1111 |             | No                 | 10/10/2017           |
| Jessica | 555-123-1234 | 700 Main St | Maybe              | 9/29/2017            |
| Joe     | 555-555-5555 | 123 Main st | No                 | 10/27/2017           |
+---------+--------------+-------------+--------------------+----------------------+