“Best?” That all depends on what you like to do.
Let me answer the second part of your question first:
Diet, Diet, oh and did I mention… Diet. I follow the motto,
“It’s not a diet, it’s a lifestyle”.
A combination of whole foods and daily activity will help you reduce fat.
To your lower abs, core exercies in the supine position, such as lying leg raises, flutter kicks, v-ups, and reverse crunches are excellent moves for your lower abs.
Good luck, and stay strong,