What is the most popular blog subject ever? What category gets the most draws the most traffic? What topic generates the most revenue? Answer: Whatever is the most original at the time.
Since the earliest days of blogging, the most popular sites have always been the most original. I do not necessarily mean the most original writing, pictures, videos, or designs. Although, that certainly helps.
The most popular blogs have always been the ones with the most focused, original subject matters.
Until the early 2000's, there were no such thing as a "politics blog" or "tech blog." That meant that early political blogs like DailyKos and FireDogLake were extremely popular. Why? There wasn't anyone doing it at the time. As crazy as it sounds, a politics blog was something that would have been considered niche for a pseudo news site.
Once political blogs started to surge in popularity, they became a dime a dozen. In order to compete, sites like DailyKos had to continue to improve the quality of their content since they were now competing for a share of each user's attention.
Today, all the major subject areas are saturated with sites all writing about and saying the same things. The more niche, focused, and original your subject is, the better the chance of standing out.
It is almost impossible to launch a general interest, cooking, mommy, gossip, political, news, or sports blog and actually expect to get huge traffic.
You've got to take your subject layers deeper in search of originality.
If you want to start a general sports site, don't. Instead, pick team. Pick a sports town. Pick a player.
The more niche you are, the easier to make a name for yourself. The tent will be much less crowded while you get up and running. This will help you actually get the initial traction you need to build an audience.
Speaking of audience, don't be afraid that being too niche will limit your audience. First, you would be surprised how many people are searching specifically for content related to that niche. Secondly, users appreciate a site with clear focus. They like it so much that they come back often.
Overall, it doesn't matter what subject matter you are passionate about, just pick one. Then, dig deep. Take ownership of your own little corner in that subject.
Be original. Do it well. The traffic will follow.