For beginners, 'Saga' could be a great choice. It has a very accessible story with elements of adventure, love, and family. The art is colorful and appealing, and it doesn't get too bogged down in overly complex science - fiction concepts right away.
Maybe 'Y: The Last Man'. It has a simple yet fascinating premise - what if all men but one died? The story is easy to follow and it shows how society would change in such a drastic situation. It's a good starting point for getting into science - fiction graphic novels.