It depends on context. Both of them CAN be correct. The most amazing experience I have ever had ... means it is STILL the best thing you have done in your life The most amazing experience I had ever had ... means you have since done something better. Living in China is the most amazing experience I have ever had. Before that I lived in Korea which at the time was the most amazing experience I had ever had.

"It was the most amazing experience I had ever had" is correct! If the event is still going on you can say "This is the most amazing experience I have ever had", but if it has passed you have to put "to have" in the past as well.