It sounds like your friend may be dealing with a psychological issue, something like depression or low self-esteem. If this is the case, then she is not trying to prompt a reaction from you. There isn't anything in particular that she "wants to hear", but rather you are experiencing an expression of the underlying issue.
These kinds of issues aren't necessarily ones that you are going to be equipped to tackle in arbitrary social settings. They are more in line with what a professional therapist or psychiatrist can provide. Even if your friend is fully aware that your are her friend and you care that won't necessarily move the needle on her thoughts or behavior-- she may not believe those things in the moment (even if she does at other times or rationally understands that they are true).
What you can do:
Please note that these are suggestions from my personal experience and are definitely not a comprehensive list, nor are they things that I would expect to work for every person every time. And, importantly, they shouldn't be expected to fix things so much as maybe improve them.
1. Reassure, but don't participate in wallowing
It's great to say that you like her art, and that's a valuable contribution in that it does not feed smoothly into spiraling thoughts of low self-worth. There is some risk in your responses becoming mechanical, especially if the negativity is common for your friend, and mechanical responses will be less reassuring to your friend and easier for her to dismiss. I can't tell you where the right balance is, but I would say it's best to be clear and decisive: "well, I can't speak for you, but I really like your artwork [and maybe something specific that you like, some technique or a good piece]".
But try to avoid getting sucked into intricate details, which may be loosely connected (at best) to her feelings and more justifications for them instead. Your consistent message is that you do not share the negative opinion that she is expressing. You might not have new things to say, but she's not feeling a new thing. You are not building a rational case to convince her (which probably won't work), you are countering her non-rational sentiments.
2. Provide opportunities to do something other than wallow
This one can be hard, but sitting around and not interacting (like at a movie) gives a lot of opportunity to close of and focus only on negative thoughts, which can build on one another. I'm not saying that movies are bad, just that they are not necessarily ideal for addressing these issues. Activities where there are things to do and see and, importantly, interact with external things, might help her focus on other things.
3. Engage her
She sounds like she's having trouble reaching out to you, especially when feeling as you describe. You can help by contacting her yourself (planning things to do and inviting her, following up and encouraging her to attend), and then interacting with her when you're together. A good goal is to get her out of her own head, and external activities and conversations are good for that.
4. Don't force things for no reason
If she doesn't want to pick the movie, don't make her, and don't dwell on it. It's probably better for her to come along and engage socially with you all instead of having her do that plus adding the pressure of pleasing everyone else with a (to her) high-stakes decision.
5. Give examples of reasonable self-criticism
She might feel that she is uniquely unworthy, and so anything she picks will be bad while anything anyone else picks would at worst be OK. If you pick a movie, and it's terrible, a passing comment like "wow, that really wasn't great. I should have chosen the other one!" without dwelling on it can reinforce that that sort of decision is not very important and that anyone can make a non-ideal selection. It's probably not as good to point those things out about each other, as that suggests that people might judge her choices. But if you pick a bad movie you can show that it's a pretty minor issue.
You might even be able to encourage her to be more assertive. If you're getting something to eat afterwards, something like "well, I'm not lucky at picking things today, so someone else should choose the restaurant. [Friend], where should we go?" can indicate that even choices that aren't huge successes are OK, and that she would not be alone in making such a choice. But, as above, if she really doesn't want to, there's no real reason to make her.
6. Professional assistance
Like I suggested above, there are a lot of situations that would match what you are describing that are not suitable for social friends to address on an event-by-event basis. A professional can be very valuable in those cases, if she's open to help.
I go through depressive cycles myself, and have for my whole life. Some of what you describe in your friend resonates with me (though that in no way means that the situations are the same; I do not know your friend). These are largely drawn from my personal experiences, particularly my reflections during non-depressed periods.