You asked your coworker to cover your shift at your part-time job, and he agreed. You're really grateful. You say:
Thank you SO much! You're the greatest!
This is what you say to someone when they helped you out a lot and you're really grateful. You pronounce this phrase with strong stress on the word "so".
This is something else you say when you feel grateful to someone. It literally means "you're the greatest person I know" but of course isn't meant that way seriously. You can say this to people in work situations as well as at home and with your friends.
You might also hear another similar phrase:
You're the best!
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