The ones you will never rank for. Go for higher paying niches, but forget the top ones. They are way over saturated and unless you have an authority site that is better than anyone else and converts for the advertiser, you will likely get smart priced and wind up with clicks paying a few cents.
the highest paying niche in my opinion are those which have lot of traffic coming from non-technical people. That way the ratio money/time spent is way better.
Depends on how much you're trying to find. Health, forez and IM are always at the top but its harder to rank for the more competetive keywords. If you drill down you'll find some easier keywords but the click rate would be lower.
financial, investment, weight loss, forex, and internet marketing or just marketing in general rate high, but they are all almost impossible to actually make any money in, as the cost per click is also high ranking, so I would look at something else if it were me and I have, but only you know how much money and time you have to invest in a questionable venture, but good luck.