Hello flexpaydude,
In 2025, the least effective technique is definitely automated keyword stuffing and thin content generation that lacks a unique perspective. Google’s algorithms have become incredibly efficient at devaluing pages that don't satisfy Helpful Content criteria or provide actual E-E-A-T signals. Relying on low quality mass produced guest post spam is also a fast track to a manual penalty or, at the very least, a complete waste of crawl budget as those links are now frequently ignored.