These are the top Tumblrs in terms of web traffic, according to Quantcast. Keep in mind, web traffic is different measure of popularity than followers; followers view postings from within the Tumblr dashboard as opposed to visiting the actual webpage.
Update: A commenter pointed out that these are the rankings for are for subdomains of tumblr.com only, and exclude blogs that run on tumblr but use their own custom URL.
The tumblr for the largest Argentinian newspaper, Clarín.
Photos of white Silicon Valley bros wearing Google Glass.
14. Old Love
Photos of long-lost celebrity couples.
Nude and explicit scenes from mainstream movies (NSFW).
12. Back on Pointe
A Tumblr of inspirational quotes and photos about fitness from a college dancer.
Comics based on ridiculous tweets.
Photos of crying kids along with an explanation of why they're crying submitted by parents around the world.
“We said she couldn’t have more bacon.”
Comedy Central's official Tumblr.
Reaction GIFs for everyday situations people frequently find themselves in.
"When my mom wants me to help out around the house."
Images and GIFs from movies and TV shows.
Screenshots of offensive tweets taken by a guy named Matt Binder.
A newish Tumblr that collects depressing Craigslist apartments in New York City.
4. The OneNaBle
A Korean comic site.
The Tumblr of David J. Peterson, a linguist/language creator who made the Dothraki and Valerian languages for the HBO show Game of Thrones. A large percentage of his posts are answering fan questions about the languages.
A Brazilian site of reaction GIFs. It means "how I feel when" in Portuguese.
"...When my boss catches me on Facebook"
The official news update site for the popular game Minecraft. Most posts are about various technical updates.
Thanks to Ant de Rosa for suggesting the idea for this.
Correction: The Clarín tumblr is associated with the Argentinian newspaper of the same name. A previous version misstated that fact.