Depends on how you say it and in which context.

My Bad-to-Good spectrum would be:

Bad

Pretty bad

Fairly bad

Fairly good

Pretty good

Good

For me, it all depends on tone of voice, and what word is emphasized. “Pretty good!” generally means better than expected. “Pretty good” is like “Good, but…”

Neither, really. I use it to mean "more than expected". Like, if my kids insist I watch a cartoon with them, and I enjoy it, i'd say it's "pretty good". Or if a coworker said something awful about a customer, I might say the comment was "pretty mean".

it can mean "decent, better than I expected"

or

"good, but not exceptional or memorable"

Generally better, but it's contextual. Someone saying with a surprised tone "It was pretty good" implies that it's better than good and better than expected, and that the person might not have expected it to be good. Someone saying with a muted or dull "It was good..." would be worse than "pretty good" and implies that it was acceptable but disappointing or mediocre.

Worse.

See also: "Pretty close"

I'd say it means goodish

Depends on the tone.

I think it depends on what expectations were.

You and your friends make a horror movie and it's better than you thought: Hey, that's pretty good.

Steven Spielberg makes a mediocre horror movie: It was pretty good.

Pretty good is less than good.

I think I use 'pretty good' to pretend I have an opinion about something, rather than it being a moral judgement:

How was the film? Pretty good vs.
How was the film? Good.

Depends on tone.

Intonation can carry the meaning.

In the UK, pretty good can mean anything from "not as bad as I thought it would be" to just "OK" or "pretty good! :D"

It would totally depend on my enthusiasm. More often, though, it means worse.

Worse: "Yeah, it's pretty good. But..." <- Talking about a game or a movie

Better: "Whoa! This is pretty good!" <- Food!

I see “pretty good” to be better than just “good”. It seems obvious to me, considering one is only good but the other is pretty good.

I can't imagine how "pretty good" could mean "better than good." Most of the examples posted here are talking about how something relates to your expectations, but that's not the question. Yes, "pretty good" is often used to describe something that is better than expected, but that doesn't make it better than "good."

For example, it doesn't make sense to say "$50 is good, but $100 is pretty good!"

I do think "pretty good" is often used as an understated way to say that something is very good, e.g. "Yeah, Messi is pretty good at soccer." However, that's a play on the actual meaning of the phrase, and should not be construed as the actual meaning.

I always thought pretty good was better than good. Honestly surprised to find out I may have been wrong my entire life.

Good

Pretty good

Really/Very good

Extremely good

Great

Pretty great

Really/very great

Extremely great

Awesome

...

Worse.

Not necessarily worse, but more ambiguous than if I say something is good.

Good>pretty good

The word good is a stronger affirmative imo

Better

pretty good is less good than good

Worse. Sometimes just a little worse than good, sometimes it's medium or negative but you're just trying to be nice. "How did you like the liver and onions I cooked for you?". "It was pretty good."

It's not just good, it's good enough!

I learned about it the other day. Apparently native English speakers know than non native use it in a more positive way then they do

When we say something the words have more impact than the word itself.

Generally, for me, it means something less than entirely "good."

The times I'm most likely to use it are when I'm finding minor fault with something - "Well... it was pretty good, but..." or when something is better than I expected, but not quite fully good - "Hey! That was actually pretty good!"

Slightly worse

https://youtu.be/O_05qJTeNNI

Pretty good is not at good as good.