Which one is best scope identity or get the maximum id in SQL server?

Never use maximum value to assume an ID for uniqueness - in a multiuser environment, it will give intermittent problems which are a pain to work out, much less fix. Always use scope identity, but make it part of the INSERT - i.e. write a stored procedure to do the insert, and return either the ID value or an error code. Never try to do it as two commands from your C# code.
The alternative (which I prefer) is not to use IDENTITY fields at all, but to use GUID ID's instead, via a UNIQUEIDENTIFIER column, and assign it from your C# code when you do the insert. That way, you are in total control of the values and don't need to "look up" what value was inserted. I only use IDENTITY fields to provide unique values that I don't care about, and won't cross reference such as log table IDs.

As for your "deleted rows" problem - without the relevant code fragments we can't have any idea! :laugh: