What is the BEST WAY to write test cases?

Answer1:
1) List down usecases (taken from business cases) from functional specification. For each use case write a test case and categorize them into sanity tests, functionality, GUI, performance etc. Then for each test case, write its workflow.
2) For a GUI application - make a list of all GUI controls. For each control start writing test cases for testing of the control UI, functionality (impact on the whole application), negative testing (for incorrect inputs), performance etc.

Answer2:
1. Generate Sunny day scenarios based on use cases and/or requirements.
2. Generate Rainy Day (negative, boundary, etc.) tests that correspond to the previously defined Sunny Day scenarios.
3. Based on past experience and a knowledge of the product, generate tests for anything that might have been missed in steps one and two above. These tests need not correspond to any documented requirements or use cases. It's generally not possible to test every facet of the design, but with a little work and forethought you can test the high risk areas or high impact features.