Sales copy is definitely the best type of content. Copywriting is much more lucrative than broad content writing. Niche content writing is one step forward, read on to see why. Of course content that is in volume, is cheap and also good is in most demand. However, this type of content became commodity - you can sell it like a peddlar and someone will buy - yet the price won't cross 2 cents per word. Since demand for it was so huge, people add it to carts ignoring specific attributes. In fact, most sellers will just ask for a list of keywords which for professional copywriters is a big red flag.
So I would put it as follows, ignoring the question somehow and simply focus on long term success.
1. Sales letters, brochures, mailings. Requires C2/native. You need to be passionate about English to get there. If you don't like learning new words, say no to this.
2. Specialized materials like case studies, landing page copy, ad copy. Still if you miss something in these nobody will want you. Both this and point 1 must be perfectly optimized for target audience.
3. Specialized content writing ie. if you are an engineer, you can write great content about building stuff like me. Can be done by people like me who only speak English, have some domain specific knowledge but had no occasion to master the language itself.
4. Generalized blog posts, product descriptions, 2nd tier of everything listed in points 2 and 1 (like case study for small business).
5. SEO stuff people write when they don't know what they're doing except they've heard about that SEO thing and keywords being important. So here you have all AI content, scraped, translated and outsourced stuff.
There were couple BSTs in the past that looked more like point 4 and 3 but members went aloof. Now it's mostly 3rd world trying to get into IM. So point 1 can get you $1 per word, point 5 can get you 1 cent per word. You will sell point 5 super easily and point 1 super hard unless you are bachelor degree English speaking country student.
