I would use "What was your most memorable moment in life," because the moment is in the past, although the memory is present. But, one could say, "What is your favorite memory of an event in your life," because the memory is in the present, although the event is probably in the past. Or, you would say, "This is the most memorable moment in my life," because one is talking about this moment in the present.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.