It's been a while, but we have always loved the low lighting and "riverside" atmospere at the San Angel Inn in Mexico/World Showcase.
We also find LeCellier relaxing. However, all it takes is a large group of excited youngsters to remove the "relaxing" from any ambience, so we just roll with it at WDW restaurants.
Altho Coral Reef was not our favorite meal, it was a VERY calming and relaxing atmosphere, once the fawning waiter left us alone.