"Shut up!".
The absolute arsehole at theatre group; proper teacher's pet, telling me to shut up after a couple of my friends had been talking to me. I wrote him off as an arsehole, went home and declared to my mom that I liked everyone at theatre group except him, he was a twat and I wasn't going to get on with him (in fairness, I was 13).
You can see where this is going...
"It's a girl! We've got a daughter!".
Said by that same arsehole to me six years later
followed two years later by...
"I do".
All three of them changed my life :) other sentences include -
"I'm not supposed to tell you this until the letters come out tomorrow, but you got a First!" - finding out I'd got a First in my degree despite being pregnant for second year and a new mum for third year.
"I love you" - the first time my daughter told me, unprompted, that she loves me.
There are sad ones too, but I love my happy ones.