which one is good for generating traffic seo or sem?

Both will generate traffic to your website but SEO is better as it gives organic results in SERP's

If you have brand SEO give lot of traffic and SEM also give Lot,but SEO is unpaid, and SMO is paid,
If you haven't brand SEM give more traffic

According to my experience both can help you. But you know what is better for you and effective.

Both SEO and SEM are equally important to generate traffic to your website. They help to promote your website in various search engines to build ranking and traffic to your website.

I think seo is best because sem is short term process for getting traffic. Seo is organically long term process.