What is your favourite linkbuilding stategy?

Spying on a competitor's site using ahrefs or SEMRush and steal his strategies...

Ideally we are working with an auction domain that already has a decent anchor profile that is focused on branding, but any website, we usually start with the following:

Phase1 : WE always start with Citations (structured and unstructured) as they are quite affordable and work well for any type of site, followed by branded outreach, a good chunk of manually created and branded Web 2.0s, a bunch of the typical social profiles + Tier2 to the web 2.0s and a selective amount of citations.
Phase2: Niche PBNs, Guest Posts, Niche Curated Links, Resource Links, Editorials, HARO and sometimes Guestographics + selective high quality Tier2 to the PBNs and outreach links (if approved by the webmasters)
Phase3: Scale everything from Phase 2 until rankings are achieved.

Web 2.0

Guest Post as usual and along with involvement in Q&A websites.

The best legitimate way should be asking other webmasters to exchange links