Best is relative...if you are looking for one of the most expensive, try Serena Beach Hotel. If it is quality service Serena have some quality service, but so too do Nyali Beach, Sun n Sands and many others. My advice to you is to try South Coast especially Diani and Ukunda where many quality hotels are door to door to each other. Note though, the best hotels in the Kenyan coast do not advertise (ask Hollywood movie stars, Jose Morinho and Bill Gates) , the rely on word of mouth from patrons or referrals from tour firms, so talk to your tour agent depending on how deep your pockets are.