H2 would be best in a traditional HTML4 docuement, but in HTML5 each sectioning element has it's own document outline. The reason for this is to encourage you to use H1's for the most important content within each section.

In this case the most important content of the <article> section is the title, so an H1 would be best used here.

The point of this post is to show that by using this new guideline, it leads to problems if search engines are using the old HTML document outline.