Uniqueidentifier vs. IDENTITY vs. Material Code——哪个是主键的最佳选择?

Which one is the best choice for primary key in SQL Server?
There are some example code:

唯一标识符

例如

CREATE TABLE new_employees
   (employeeId   UNIQUEIDENTIFIER      DEFAULT NEWID(),
   fname      VARCHAR(20) )
GO
INSERT INTO new_employees(fname) VALUES ('Karin')
GO

身份列

例如

 CREATE TABLE new_employees
 (
  employeeId int IDENTITY(1,1),
  fname varchar (20)
 );

 INSERT new_employees
    (fname)
 VALUES
    ('Karin');

[材料代码](或商业代码，材料的标识.例如客户标识符)

例如

CREATE TABLE new_employees(
    [ClientId] [varchar](20) NOT NULL,
    [fName] [varchar](20) NULL      
 )

 INSERT new_employees
    (ClientID, fname)
 VALUES
    ('C0101000001',--customer identifier，e.g.'C0101000001' a user-defined code.
     'Karin');

请给我一些建议，从三种类型标识列中选择主键，或其他选择.