9- "I am so glad I met you." Because it shows their love is so deep but simple. They don't need grand gestures or poetic speeches to show how much they love each other.
Donna- "He deserved it... No, he didn't." the moment Donna stole my heart! Forgiveness is important, especially to your enemies, and this shows Donna is a very strong person, willing to forgive even after getting emotionally shot in the heart.
Rose- "Yeah, but being stuck with you- that's not so bad." See 9's description (even though it applies to 10). I related to it, relationship-wise.
I have 2 favorite characters, so they'll just share :D
Martha's- "I spent a lot of time with you thinking I was second best, but you know what? I am good." - Inspirational af. That's one to go on the pin boards.
Reinette- "One may tolerate a world of demons for the sake of an angel"- It's practically a cliche at this point, but it still does ring true today. And the way she says it!
These aren't my favorite quotes, but they're my favorite from my two favorites.