There are a lot of things that people like to hear said to them:
"You're pretty"
"You're funny"
"You're smart"
"You're nice"
"I miss you"
"I love you"
All of these are great, and they make you feel wanted, needed, and appreciated.
The phrase that has always hit me hardest, however, has been "I'm proud of you."
Someone saying that they're proud of you implies that, not only are they impressed by what you do and who you are, they also want others to know it.
The first time I remember hearing it said to me (though I'm sure there were times before the instance I'm describing) was when I was seven years old. I broke my arm on the playground, and, rather than crying, calmly walked to the teacher and told her that my arm was broken, and that she needed to call my dad. My dad came and picked me up, and it was only then that I started crying. I remember him looking down at me and telling me he was proud of me. And just then, it seemed okay to have a broken arm, because my dad was proud of me.
More recently, in the spring of my senior year I was dealing with a complicated situation. After talking through it, I felt like I really hadn't resolved anything. Standing with my friends relaying the whole story, I was feeling pretty crummy. I really had believed I would be able to work it all out, all on my own. But the two of them stood there and listened, and when I was done, one of them wrapped me up in a big hug and just said, simply and naturally, "I'm proud of you. You did the right thing."
Just this last week, as I was leaving to head back to school, my parents told me that they were proud of me for my scholastic achievements over the course of the last semester, and that they were proud of me for making good decisions.
Now, I got a lot of "proud of yous" during graduation, and during my first weeks away at university. But that's kind of to be expected. Everyone knows you need a little boost during those times. They're scary and wonderful and strange, and it really helps to hear that somebody recognizes what you're doing right. But, at least for me, there is something truly special about hearing that the people close to you are happy and honored to be close to you on a regular day, for little things, too.