How profitable your contents will be depends to a large extent by your how 'big' your followers and network is. How 'big' is not only about the number of followers but also the steem power they have in their Wallet.
That is why it is always important for a newcomer to spend time and efforts to build his/her network first before expecting big reward on the blockchain. And this can be through the power of quality commenting.
That said, there are topics I have seen whales and minnows take interest in. If you take time to write a great comment in these topics, there is a chance one whale will see it and bless you with a sumptuous upvote. In no particular order...
I know a few number of whale that take interest in that subject. Write about some secret agency, FBI, CIA and Other American hidden agenda, human cloning, secret file, illuminatti etc.
We all love steem, at least we are here. Write something juicy about steemit, dApps, STEEM, SBD or a new interesting project on the blockchain and you will likely get some upvotes.
SCIENCE & TECHNOLOGY
We are witness to the wonder and product of science and technology. Write about some new interesting ideas and project. People will likely find it interesting and there are a couple or so community to will curate it if it is well written.
PHILOSOPHY & PHYCOLOGY
Since these two have to do with our interpersonal and intrapersonal relationships and existence. You will definitely have no shortage of readers if your posts are well written and formatted. You may be lucky and one of your devoted reader is a whale.
Who doesn't love some dose of romance every now and now. If you can write good romantic contents that will make people come back to your blog for more. Then go for it.
The bottom line is whatever niche or topic you are writing, give your reader your best, make your post well formatted and beautiful with good relevant images. Remember to reference every outsourced content.
So I took a quick look as to what Categories/Tags got the most Vote Value(SBD) last Oct 09 to October 14 and found out that the top category with a decent amount of vote value given is the "STEEMHUNT" tag followed by Tasteem, Life, and Information War which deals about controversial topics like politics , censorship, etc..
Going back to your question, it seems contents about LIFE and INFORMATION WAR (Truth, Liberty, Freedom, Wisdom, Philosophy, Family Protection, Anti-Censorship, Decentralization) are currently the most profitable one atm :)
The kind that get those upvotes, yeah!