No sure about highest but i guess Expedia.com ,Booking.com: The largest selection of hotels, homes, and vacation rentals and Hotels.com | Cheap Hotels, Discounts, Hotel Deals and Offers are the best affiliate programs in their niche.
Some other popular travel affiliate programs are
Cheap Hotels, Flights, Vacations & Travel Deals
Compare & Buy Cheap Flights,Hotels,Holidays & Travel Deals
TripAdvisor: Read Reviews, Compare Prices & Book
All credit goes to Google, I am personally using Expedia and Tripadvisor