A lot of words have power. Here are some of the ones I know, and why I think they do.
I am-A declaration of self. On its own, it is fairly powerful, simply because it is saying that the person is. That's one thing another person can't stop, not completely. Someone can kill you, make it into, I was, but in thoughts, in help, in joy, in inspiration, the ripples move out. With a description after it, it becomes more powerful, because it is not just saying, This person is real, it is giving a way to describe. I am a scientist. I am a singer. I am a child. I am a woman. To reinforce both, I am human, I am a person.
I believe-As intimate as I am, but to do with the outside world. This doesn't need logic, nor its absence. It is just a belief. And that'll move mountains.
Especially powerful in, This I believe. The archaic diction gives the phrase some power in itself. Things that have survived this long have survived for a reason. That reason does not always apply in the here and now, but it was there. That sort of belief is why religions claim to predate x or be able to trace this ritual back to y. It's also why stores say Since 1948, or 1904, or 1827. There's power in old things, even if it is only the power people give them.
I know-Alone, it can fall in the same category as, I trust you, or be dismissing another person, or be a, Go on, I understand. If it's, I know myself, it means the person has a strong sense of self. If it's, I know you then it means the person has a connection, and knows what the person will or won't do. How do you know I won't? I know you, Kelsey. And then, the simplest meanings, I am familiar with this, I believe this strongly enough that thinking it false does not (regularly) occur to me.
I love you-Ah, yes. Gotta love the classics. These can be the most important words ever spoken, ever heard. Or they can fall completely flat. The problem with these words is that everyone who is even vaguely familiar with the culture will know how powerful they are. But...they're words. What matters with words is the communication of an idea. When something is so well-known already, it can look like the words are just there to have something there. Can get especially glaring when it's a response. Hm? Oh, yeah, I love you, too. Even Han's response is more traditionally romantic than that, because no etiquette class will teach you to respond to, I love you, with, I know. So he thought about it.
To go back to what originally made it powerful, the phrase can also be someone saying what one or both already knew, if not on the level of the mind that deals with words. Add a finally in there as necessary.
I think-Similar to I believe, though less firm. But this flexibility is a power in itself. I think this true, but I am not challenging you. I am telling you what I think. What do you think. I think so, emphasizes opinion, I think so, lack of certainty, I think so, certainty. Simple words with strong tone, cutting through any random jumble.
Names-There's a mythology in every culture I have run across about true names. It may not be of importance to humans, but there's someone or something out there, and if you know the true name, then you have power. This holds true in the turning-ones-life-upside down fashion with ID cards, social security numbers, etc. But there is a power in just using someone's name, even if it's common. If your parents say, JOHN SMITH! then even though that name is common and has been worn by many for ages, it still has power if it is yours. In the right tone, a whispered, John, would speak volumes.
Any words to add?