Gutenberg Hub
I would argue that Gutenberg Hub is probably the best place to find block themes at this time. There are a few features that are really helpful, in my opinion. First, they show the color palette of the theme in the bottom corner so you can easily see it’s defining colors — out of the box.
When you click on a theme, you can immediately see a preview of it as well as a rating (if anyone has tried it yet, it’s still early). Of course, the download button is out in front and easy to access.
As you scroll down, the first section you see is about the theme’s typography. You can see a preview of the fonts the block theme uses.
If you scroll down, you will see a preview of how buttons look in the block theme, as well as how tables look.
The next two sections display how quotes look, as well as lists.
The final section shows the styles available for images.
Fullsiteediting.com
Another helpful place to find block themes is over at fullsiteediting.com.
What I really like about the theme listings on fullsiteediting.com is that all of them are available in the WordPress Theme Directory right now! You can search for any of these themes and download them all within WordPress. Best of all possibly, they are all free and can be used out of the box.
Honorable Mentions
While not directors, per se — there are a few block posts nicely highlighting a handful of the block themes available.
Passion WP
Passion WP has a nice article, “10 WordPress Block Themes for Full Site Editing”. They give a well-explained overview of what block themes are, and the benefits of using them.
In Conclusion
Hopefully as full-site editing takes hold in the WordPress community, there will be more helpful places to find block themes. As I find more block theme directories, I will be sure to add them here.