personally i've found it quite easy to get sites u[ to around 150 visitors per day, its getting over that that seems difficult!
There's thousands of niche available and have the prospect of getting good traffic if you managed to get top ranking for it's keyword. You can use wordtracker's keyword tool to check for whatever keyword that comes into your mind, and make decision on which keyword you should work with. But you have to bear in mind that niche which is familiar to you or you are well versed to it is the best keyword you should work for.
Perhaps the question should be like this: tell me the easiest niche that can attract 500 visitors daily without sweat? The true is all niche can bring you 500 visitors daily, it really depends on your hard work and insight about it. If there is a easiest niche and someone have found it, the chances are no one would tell you.
Any type of humor site and put it on stumbleupon if it's good you will get a few thousand visits a day from them. i get up to 6,000 to 10,000 views a day sometimes more last week i had a day 17,000, feb. 9th I had 51,000 with 27,000 unique visitor's.
the most profitable niches are: Health, Wealth and Love if you want to really find niches that are profitable and not saturated use this tool Micronichefinder