I would say that any niche with good monthly searches is a good niche for adsense....even if the niche has a very high comp level in google you can still find other ways to get visitors to your site and clicking on you ad
As long as the search volume is high and the competition is low, you can create a site around it and make some money. Every visitor is also a human being who have basic needs like making money or dieting. You can target demographics as well and not just the niche.
insurance, fores, loans are best..but competition for these keywords are very high, specially when you want to do SEO...
Sometimes the off the wall niche sites are the ones that get the dedicated traffic that really click on ads, because there is no sites that pull the information together correctly. I have recently had a site that started earning $8-10 a day with only 4 pages of content and 5 months old.
Take any niche which you love and can be comfortable with all the time. Consistent updating and unique content along with visitors is the main criteria for getting it approved i think.
I'm into a lot of different niches... I once read where somebody wrote, "If I have to put up 1,000 sites that earn $1 each per day, then I had best get busy." Just launch sites that you know you can rank. Traffic is more important than niche. Best of luck to you.
You may want to research on the keyword and competition. Also, u must be good on seo to outrank the competition.
Every niche now is overcrowded so I think you should try first with what niche you are interested in the most.