Ahrefs is widely renowned, and near-unparalleled. However this quality comes at a steep price. At the core, they allow you to analyze your competitor's site, learn what factors are effecting their ability to rank on Google, and therefore what you can do to compete.
This is primarily done with backlink analysis, but Ahrefs includes an overall competitor analysis, along with keyword, and content analysis. You can analyse your competitor's top pages, and even compare their top keywords with the keywords on your own site, to identify gaps in your content.
Ahrefs also includes an advanced keyword research tool, identifying search volumes and cost-per-click.