My definition is between 1,000-100,000 active double opt ins. Usually for ultra specific niches I can pull in around 6 figures for 3 months straight. The key to doing this is licking people's butt and offering them outstanding psychological motivation to buy and keep buying. I can't elaborate more on it because it is a very ultra specific niche which involves 11 months or more of pure labor to yield that amount of money for that period of time. For normal people you can pull in steady mid range 5 figures with your own product or service. A friend of mine built websites for private emulation servers of popular games and pulled in around $300,000. How? He simply created a awesome site then literally licked their butt, customized it, did more butt licking, charged $300 a site with 1,000 p servers span 7 games, had people copy him, then finally made more money simply by licking more butt. Of course his audience type was 13 years old hoping to get their WoW on as admin of their own worlds but that does not water down the fact he could pull that in. Now being legal or not I have no clue, but he was pretty damn rich and inspired me to look for a method like his. Sadly, I had to go greyhat because I didn't find a method like his or a audience.
My final thoughts on the lists that make a lots of money are for niches that have people buying the most out of them. Think about games, movies, seo, household stuff, cars, etc. Think about what you can do, how you can do it, and then go for it. I went for it in 3 niches and think "shiz, if I wanted easy money I'd rob people or go work at Mcdonalds". Well, then I wouldn't be here offering advice because for drive thru there is no time for WiFi.