In your opinion, what are the ten most useful words to know? Please don't include things like "a" or "the", I'm talking words that can enhance your conversation. Also, try to avoid words that are only used in your job/field (but you can use job-related words that have migrated to common usage if you so desire).

As for me...
10. undoubtedly
9. luminescent
8. intuitive
7. serendipitous
6. superiority
5. optician
EDIT: After nancyk's post, I am changing this one to:
5. retrospect
4. blamelessness
3. sundry
2. intelligence
1. (forgive me) please

Feel free to disagree...and keep in mind, what comes in handy one day may not be so helpful the next.