seo, sem, smm or ppc, which first for a brand new business?

whatever you pick, get conversion tracking working before the first dollar of ad spend. without it the PPC test can't tell you which keywords convert, and that's the whole reason to run it first.

If it's a local business, the Google Business Profile before any of them, it's free and it's what shows up when people search near them. After that, a small PPC budget first if you need customers this month. It also tells you which keywords actually convert, which is exactly what you want to know before spending months on SEO. Start SEO in parallel because it takes the longest to kick in. Social depends a lot on the product, some businesses never really need it.