What is the most Important SEO task (with or without Add-on) with the biggest impact on ranking?

Hi, I was wondering which experiences you guys have made in the past in regards to ranking thru SEO. There are many SEO related addons but which of these really had had an impact on your rankings and how did you achieve it?

The usual SEO tactics like Title, H1, Image Name , desc are clear to me but it gets tougher when there are also several local stores to pick the items up from. Tags on product page seem quite crucial for internal backlinks and Google.

What recommendations can you give us marketplace owners?

Lets discuss