"Publishing amazing content" seems to be a favorite for many, however I'm not actually sure how many could prove success having practiced what they preach.
Judging by the questions being asked on the forum, my most underrated SEO tip would be KW research.
I've seen a lot of questions like this being asked:
How do i rank?
How do I build backlinks?
How many backlinks will i need to make page 1?
The truth is that many of the people asking these questions will be fighting a loosing battle from the start.
The landscape they have chosen to compete in via their niche & KW selection, is simply too competitive for their budget and knowledge and they have no chance to rank & profit.
Many industries/KW's are dominated by huge authority sites that will prove impossible for the smaller guy to compete against.
It's important to avoid them & take time to uncover less competitive niche's that still have volume and also buying intent - there are plenty available, you just have to work hard to find them.
What's your most underrated SEO tip?