What are the best XP practices?

Question

In "Extreme Programming Explained", Beck lists 13 "primary practices". They are:

• Sit Together
• Whole Team
• Informative Workspace
• Energized Work
• Pair Programming
• Stories
• Weekly Cycle
• Quarterly Cycle
• Slack
• 10 Minute Build
• Continuous Integration
• Test-First Programming
• Incremental Design

Which of these have you actually implemented in your workplace? Which has been the most useful?