Last Confession Wins

In Unwanted Harem shows, the last girl to show or confess her feelings for the lead is the one with the best chance of winning him.

Usually they will look like the least compatible couple in the beginning; she will only warm up to him later in the series.

This law is especially likely to hold if the girl who will be last to confess is a Type A Tsundere.

Note that it's the last confession that wins. This rule and First Girl Wins can hold at the same time, and it is entirely possible that everyone else will notice her love before she can admit it.

Corollary: The girl who states her attraction to the lead from the get-go stands next-to-no chance of winning the guy.

The reason for this is simple. The girl in question is the one whom the protagonist will love back after the confession, the other half of the Official Couple. Once the official couple is truly official, the premise for the Unwanted Harem show is gone. Thus, her confession will be the last for the same reason people always find lost items in the last place they look: after that, there's no longer any need to look.

"Winning", in this sense, tends to be spoilerific, so

Anime and Manga

 * Ranma ½ is a classic example with Ranma and Akane.
 * Although technically, Akane has never actually confessed that she likes him. (It's pretty damn obvious though.) Ranma is the only one who has officially confessed out of the two of them (He denied it pretty quickly afterwards, but hey, it counts!), so this example is inverted.
 * The same goes for Love Hina with Keitaro and Naru
 * The Corollary is painfully demonstrated in the Da Capo II anime - Koko confesses her feelings to Yoshiyuki at the end of the first episode. While he accepts and they go out for awhile, their relationship is rather strained, and.
 * In the SHUFFLE! anime, it is played straight with.
 * Ai Kora has Sakurako announce her love last. In fact, she doesn't confess until a little over halfway through the very last chapter of the series, even though Maeda had confessed his love (several times in one day!) in an earlier chapter.
 * Averted so far in Naruto. We've had two confessions. The first doesn't win; The second wishes it were the first. Note that even though it's been years of real-life time since these events transpired, the related plotlines can be seen as not quite resolved yet, depending on which brand of Shipping Goggles you're wearing.