Different ways in implementing CompareTo()

Following are the three different ways of implementing CompareTo(). Could you tell me please which is the most acceptable to programmers and why?

1)
public int CompareTo(Object o)
{
int returnVal;

Employee temp = (Employee)o;

if (this.idNumber > temp.idNumber)
returnVal = 1;
else
if (this.idNumber < temp.idNumber)
returnVal = -1;
else
returnVal = 0;

return returnVal;
}

2)
public int CompareTo(Object o)
{
if (o is Employee)
{
Employee temp = (Employee)o;
return temp.idNumber.CompareTo(this.idNumber);
}
else
throw new ArgumentException("Object is not a Employee.");
}

3)
public int CompareTo(Object o)
{
Employee temp = (Employee)o;
return (this.idNumber - temp.idNumber);
}