A good "niche" is one that you know something about, can remain interested in, and can contribute something meaningful about.
low competition usually means "low cost" i define good niche like: Very good click volume, very few competing ads, decent or good search volume for the keywords. Good niche is like 4 or 5 ads with you, clicks which give you pos #3 or #4 with $0.20 rare, but they're out there!