what fats are good/bad?

IMO, as an interested non-expert, the notion of good and bad fat is mainly a simplification, a shorthand. You asked what I think. The following is the literal answer to that, what I think, and that's all. Not "facts", just amateur opinion based on reading and such.

The current mainstream expert view seems to be that trans fats (hydrogenated oils, to oversimplify) are seriously non-healthful, and should be avoided as completely as possible. There's some evidence that artificial trans fats (like the hydrogenated oil) may be worse, and that the small amount of naturally-occurring trans fats (in other food, generally dairy and meat products) are not a significant health risk. So, maybe artificial trans fats are "bad".

Beyond that, my understanding is that what matters is not so clear cut as "bad" and "good".

Humans evolved eating a mix of fats, from animal and plant sources. Over most of history, that was unprocessed or lightly processed foods, mostly. That makes me think humans can eat quite a range of fats, and thrive. (I think most registered dietitians would agree - that seems to be the mainstream dietary advice).

In our modern social context, we're not eating in the way humans evolved. That's not necessarily a bad thing in itself, but it changes our context. The mix of fats we eat (statistically speaking) has changed even over my lifetime (I'm old), let alone my parents' lifetimes (they were old when I was born).

In my understanding, the mainstream expert view is that the average person's fat intake is unbalanced, and that that imbalance may be less than ideal for health.

It's statistically common for people (in the US anyway), to eat a lot of saturated fats (from meats and dairy mostly), and a lot of Omega-6 fatty acids (in lots of things, but the big source of consumption is seed oils, I believe - canola, safflower oil, sunflower oil, corn oil are a few examples).

It's also statistically common for people (US) not to eat nearly so much of monounsaturated and polyunsaturated fats (from things like olive oil, nuts, seeds, avocados, etc.) and not very much of Omega-3 fatty acids (from things like cold-water fatty fish as well as some nuts, seeds, etc.)

An aside: Many foods contain a mix of fats, so what I wrote above about food sources is generalities, i.e., when I mention foods, they're examples of things generally regarded as good sources for the named thing, but often they contain all the things. (Example: Olive oil is 70%+ monounsaturated fat, includes some polyunsaturated fat, but it does contain a smaller amount of saturated fat, too.)

So, statistically speaking, people eat a lot of saturated fats, relatively less of polyunsaturated fats and monounsaturated fats. Similarly, statistically, people eat a lot of Omega-6 fats, not very much of Omega-3 fatty acids.

This has led a lot of people (including some experts, when simplifying) to describe saturated fats and Omega-6 fat sources as "bad" and Mono/polyunsaturated fats and Omega-3 fat sources as "good". It's oversimplified.

Further, context matters. For any nutrient, if someone eats enough of some nutrient X, severely not enough of nutrient Y, it would be "bad" (oversimplifying") to eat more X. It would be "good" to eat more Y. True of fats, as well as many other things.

The main issue, IMO, is not "bad" or "good" fats in some absolute sense, but rather that it's desirable to get a rational balance of saturated/monounsaturated/polyunsaturated fats, and a rational balance of Omega-3/Omega-6 fatty acids. (It may also be good not to get lots and lots of fats in total, because they're calorie dense - one doesn't want those calories to put us in a surplus so gain weight, or drive out other important nutrition when we try to stay within sensible calories.)

Broadly speaking, people like simple rules, clear categories, not complicated, nuanced advice (which seems like waffling). That's how we get to this "good/bad" oversimplification.

This is a special case of people talking about "good" and "bad" foods. Other than poisons, allergens, and things contraindicated by a person's health conditions or medication regimen, I don't think there are "bad foods" in an absolute sense, nor "good foods" (let alone "superfoods") in an absolute sense. Nutrition is important. Balance is the goal.

What I think is that there are better overall ways of eating, and worse overall ways of eating, on average over time. That includes calorie content, nutritional content, and more. (Humans are adaptive omnivores, but the extreme cases might even be "good" or "bad" overall way of eating. Still, most people are in the moderate middle someplace, I think, even though there are some ways in which the statistically average US diet is sub-ideal.)

One of the biggest things we can do to promote good health is to be at a reasonable body weight. That's mostly about calories. Many people see improvements in things like blood lipids and blood sugar just doing that, even without optimizing nutrition. Beyond that, it's a good plan to get enough protein, get enough fats in a good mix of the categories above (except artificial trans), eat a boatload of varied/colorful fruits and veggies for fiber and micronutrients. To me, that's good enough overall nutrition. If a person cares about their health, they'll also get some activity (maybe as exercise, fun/moderate is fine; other things can be the activity, not just formal gym-type exercise).

Another aside: Research doesn't stop. Nearly the whole point is to prove that things we used to think were true are actually wrong, or at least not sufficiently nuanced. This is not some nefarious plot to confuse us, or scientists being stupid, it's just how the process is supposed to work, IMO how it needs to work. Of course, as a result the advice keeps changing, and that will keep happening.

At this stage, when it comes to nutrition, I think we know a lot of the big stuff; the fact that humans are adaptive omnivores is a saving grace for minor issues. It's easy to fault mainstream dietary advice - like the low fat craze, for example - but it's hard to blame it for things when people don't actually follow the advice, which statistics suggest they don't. I try to follow it, mostly, while eating foods I like - seems to work OK. 🤷‍♀️

P.S. Dark chocolate is not "a must for heart health". It's fine, for heart health, but not "a must". I wish, because I'm eating some most days, would love to feel virtuous about it . . . but it's just one food, y'know?

P.P.S. Apologies for the essay . . . but you did ask.

TL;DR: Don't worry about "good" and "bad" that's too simple. Get reasonable overall nutrition at appropriate calories. That's plenty good enough, IME.