Tough one really. I'm tending to say the majors, but you have 4 chances each year to play. if you play for 10 years, you have 40 chances to win a major.
Olympics are once every 4 years. So in 10 years, you may only get to play 2. I think an Olympic Gold for your country is bigger.
Venus said that after she won. And even Dementieva, with a silver, said it will the best moment in her career, no matter how many majors she wins