Database Saveresult - How to get the id of the record where update failed?

Depending on the situation, if possible I usually use the following method for my DML error handling.

public static void dmlErrorHandling(List srList, List contactList, String additionalMessage)

{
    List successIds = new List();
    for (Database.SaveResult sr : srList)
    {
        if (sr.isSuccess()) { successIds.add(sr.getId()); }
    }
    if (successIds.size() != contactList.size())
    {
        for (Contact c : contactList)
        {
            if (!successIds.contains(c.Id))
            {
                // Handle the error, Note that you have access to the failed record
                System.debug('DML failed for Contact with ID: '+ c.Id);            
            }
        }
    }
}

Just to clarify the logic here: You loop over the List of Database SaveResult and add all the IDs of the records that didn't fail during DML, to a List of IDs. Then you loop over the initial List of records that you passed to the DML method (Insert/Update) and for each record ID you check to make sure it exists in the list of success IDs and if doesn't, there you have a record with failed DML and you can do whatever you'd like to from this point with the failed records.