Bananas are actually quite high in starch and (I think) carbs, so no, they aren't the best fruit for losing weight. However, if you're substituting a banana for a chocolate bar, obviously that's a better choice. I would definitely not recommend any diet that has you eating only one thing, or mostly one thing like the "Banana Diet." That was all the rage a few years ago in Japan when I was studying abroad there.
Funny story: My Japanese friend went on an extremely similar "apple diet" and lost a lot of weight while doing it. One day, though, she started getting mysterious and severe stomach pains that were apparently related to her only eating apples for months. After she went off the diet the pains went away, but to this day the sight of apples gives her physical pain in her stomach. Long story short: Don't do fad diets.