Here are some of the best cruise lines for adults according to the search results:

1. Viking Cruises - Best Adults-Only Luxury Cruise
2. Saga Cruises - Best Adults-Only Cruise For Older Couples
3. Virgin Voyages - Best Adults-Only Cruise For Single Adults
4. Regent Seven Seas Cruises - Best Adults-Focused Luxury Cruise
5. Seabourn Cruise Line - Best Adults-Focused All-Inclusive Cruise
6. Norwegian Cruise Line - Best Adults-Focused Cruise For Younger Adults
7. Windstar Cruises - Best Adults-Focused Cruise For Parents Who Want Alone Time
8. Celebrity Cruises - Best Cruise Line for Couples
9. Azamara Cruises
10. Oceania Cruises
11. Carnival Cruise Line
12. Silversea
13. Seabourn Cruises
14. Windstar Cruises
15. Royal Caribbean International

Some of these cruise lines are adults-only, while others have adult amenities and activities. The best cruise line for adults will depend on personal preferences and budget.