There is no part that's more interesting than another if the people in the marriage continue to learn and grow. Each stage of life that the married couple goes through is interesting for different reasons and in different ways.
The beginning is interesting because you're just getting to know each other. You're finding out new things, experiencing attraction, and exploring common interests.
When you have children you get to witness what you're creating with each other. You get to explore the highs and lows of parenthood and laugh and cry harder than you ever have before. You get to know what you each are like as parents.
When you are able to retire and live alone together once again, you get to find out how all these experiences you've had together have changed you both. You also get to decide what new experiences you'll have together and, if you have grandchildren, you get to watch your progeny and maybe even take part in their learning and experiences.
I look back with fondness at the beginning stages of my relationship with my spouse and cherish the memories we created.
I'm enjoying the life we have now together with our children, learning how to grow together and loving our experiences together as a family.
I look forward to our later years together and am anticipating more learning, growth and love in our relationship.
That's my view, anyway :)