A few months ago, my 3 yo daughter was lying next to me about to fall asleep, and reached up and took my face in her hands and said, "Mommy, your my best friend." Talk about melting your heart!
Another recent 'cuteness' was when my 5 yo daughter (who is VERY outspoken) asked me a question, and then said, "Mom, do you know why I asked you that?" I said no, and she said, "Because sometimes, you know things."
It also never ceases to strike me as totally adorable when my 5 yo uses words and phrases like "apparently" and "according to my mother."