Of the six I have been to so far, below is my list with best overall experience on top:
Aman-i-khas - This was our best overall experience, but I could see how it would not be at the top for many people (especially if compared to Africa safaris). We had a great tiger safari and fort visit
Amanjiwo - Our first Aman. Unique location, great service
Aman Tokyo - We had great service here and the excursion to the buddhist temples out of town was fantastic. Such a calming place to come home to after sightseeing all day
Amanbagh - Loved the excursions and the grounds; Room was having issues and service was a let down
Amanoi - Great hard product and enjoyed the food; found housekeeping to be a letdown and not much to do
Amangalla - Enjoyed the stay and it is an interesting experience, but not necessarily an Aman-type stay. Reminds me of Riads in Marrakesh or refurbished Cartagena boutiques we have stayed in