When Does the Anime Harem Genre Thrive?

The harem genre is a term used for anime that have one guy surrounded by a bunch of girls (usually they have a romantic interest in the main character). Sometimes there are 'reverse harems' where it's 1 girl surrounded by a bunch of guys. Take a look at the history of the harem genre in anime, which series have been popular, why the harem genre is popular/interesting and some of the most entertaining, high quality, maybe even odd harem anime. Are love triangles a more effective system?