a touchy (touching?) subject

For published authors:

What do you think is the one best promo tool you have used? This is a tool that you have control over (chats, web sites, etc.).

What do you think is the best promo tool your publisher can use (chat groups, cover art, etc.)

For non-published authors -- those of you who also read books and buy books and who are DREAMING about your first published book --what lures you to buy a book?

Blurb? Cover art? Chat groups? Banner ads on web pages?

Rank them in order of importance, most important to last.