it depends upon what you want to achieve - if you want huge traffic and good earning go for entertainment niche.They get poor clicks but HUGE traffic after some time.Or you can start a niche blog with high CPM like finance blogs.Even if you get a less traffic you make a lot woth finance blogs. But if you are a newbie it is better to stick onto what you love the most.This is because niche blogs would require you to be more patient.You need to do more work on it - like more link building.So it is your choice.
Try out checking the high paying keywords for adsense, it will give you idea for a profitable niche for adsense.
Whatever niche that pays you well may be considered as the best one but that is equally difficult to conquer, in the sense that it is very difficult to pull targeted traffic because of huge competition. Hence, it is always better to start a niche in which you are more comfortable and that helps in the long run too...
for a newbie its always advised to start with the niche he is interested in...its not just, 'money' in the beginning.
The higher the cost per click, the more competitors you will have. The lower the cpc, the lower your traffic will be. The trick is to find some middle ground.
redneck jokes... but seriously, as has been mentioned previously just go with something that you are interested in. Maybe someone can point out a few niches that arent profitable?
Although that may seem like a flippant answer, its probably correct. Lawyer sites probably pay a lot for every client referral. Understand they make tens of thousands ff of one client ..they probably pay ~100 for any successful referral.
The best subject is niche. You just asked what is the best niche. If you google what is the best niche the # are 6,830.000. google keyword tool for best niche ideas for the month of may not enough data but under additional keywords words to consider niche came up with 1,000,000 searches in the month of may so there is your niche
Its very difficult to guess what's the best niche as it depends more on how you can manage your site. Choose any niche that you are interested in and then choose some high paying keywords from that niche. Then you can try to promote the site. Its always better to choose a niche in the area of your interest as you are motivated for a longer time.