Naturally, a lot of these depend on the context of why they're said and what your relationship is like.
"I think we should take a break."
What, are things not working out?
"You're the only person I have in my life who supports me."
While everyone goes through hard times and moments when they feel alone, a good percentage of the time they're just trying to make you feel bad or piteous of them. More times than not, unintentionally or not, this is a rather manipulative thing to say.
"You're my only friend."
Refer to above.
"We need to talk."
It's the ambiguity of it and the suspense it creates that makes this one awful.
"You should make more of an attempt to show me that you love me."
Ah, nothing like a big middle finger to whatever attempts you've been trying to make to show you care, especially when they neglect to put in any affectionate attempt of their own!
"No, it's whatever."
"You're no help."
No, but this statement is super helpful. (Also, I've only ever heard people say this after they heard something they didn't like, not something that was truly unhelpful.)
"I think we should break up."
Plain and simple, it just hurts, regardless of if you're expecting it or not.
"So...my Ex just called me."
Whether you're a jealous person or not, there's just something ominous about this.
"My friends can't stand you."
"What can't you just do this one thing for me?"
It's passive aggressive and somewhat manipulative. You shouldn't feel swindled into doing something if you're uncomfortable with it.
"Come on we've been dating for # months/years now!"
No means no, whether it is sexual or not. It's never okay for people to disrespect your choice.
"I want to marry you! / We should get married!"
Contextually, this is really sweet, but if it's your second month of dating it's a major yikes and a red flag that you're with someone who moves very fast.
"You're always so busy with work/school! You never spend time with me."
No one should ever berate you for putting work or school before them. As unromantic as it is to say, the money you earn from work and the education you earn from school are more-or-less permanent or can be, whereas relationships are volatile and have no guarantee to last.
"If you loved me you'd..."
Even if they are fine, there always feels like there's a lot more hidden away behind these words.
"How would you feel if we added another person to our relationship?"
This sort of thing rarely works out for people, regardless of if they're into that sort of arrangement or not.
"No, you can't meet my family / My family can't know about us!"
The same applies if they say this in regards to their friends. It's one thing if they want to hold off for a while and get to know you more, but if it is just an impossibility for them, then you might want to continue on with a little suspicion.
"You're my one true love!"
Again, contextually, incredibly charming, but after you're second date this might be a clue to skip out on a third.
"I've been cheating on you./ Have you been cheating on me?"
It makes your heart pang to be told the truth or to be accused. It's one of the worst things someone can do to you in a relationship.