Formal:
* C'était magnifique. (This is the most literal and common translation.)
* C'était grandiose. (This emphasizes the grandeur and impressiveness.)
* C'était splendide. (This highlights the beauty and splendor.)
Informal:
* C'était génial! (This is a more casual way to say "that was amazing.")
* C'était top! (This is a very informal and slangy way to say "that was great.")
To emphasize a specific aspect of the magnificence:
* C'était magnifique de voir... (This highlights the beauty of seeing something.)
* C'était magnifique à écouter. (This highlights the beauty of hearing something.)
* C'était magnifique de ressentir... (This highlights the beauty of experiencing something.)
The best way to choose the right phrase depends on the situation and your relationship with the person you are speaking to.