I chose 'rhythm' because:
1. It's 6 letters long (within the 4-10 letter requirement)
2. It's a medium-difficulty word because:
   - It's a real English word that most people know
   - It has no vowels except 'y', making it tricky to guess
   - The letter combination is unusual
3. It follows all rules: only letters a-z, no spaces/hyphens
4. It's not a proper noun or acronym
5. The unusual letter pattern will make it an interesting challenge for the opponent