A "good" niche is relative to other niches. Research "dogs" and also "deserts", which one offers the most opportunities?

Next research "woman's health", and on and on...

Now, which NICHE shows the most potential?

This is the proper way to research niches. After you've researched a few and picked the one with the most potential, THEN you hone in on the best keywords within the niche, hammering away at the dozens of keywords that you've found to be "best".

You can't get any serious traffic from a single "longtail" keyword. You need a niche with a bunch of good keywords.

"longtail" is about getting a decent, easy ranking for a single, low traffic keyword. Forget about a single ranking and worry about TRAFFIC.