Honda cars and motorcycles.
Terry Pratchett
Star Wars, at least the original movie, was both immensely popular and a damn good movie. It was also the first time “pulp” or “space opera” sci fi had appeared on the big screen, as opposed to stuff with a serious bent and connection to Earth like 2001. It was an amazing example of world building and intentional lack of context, innovative for film.
Yellowstone National Park. It is just amazing.
The Shawshank Redemption.
Wall-E.
The iPad. Concept and implementation.
Because the thread topic is “Immensely popular things that really are good.” If the Harry Potter books aren’t good, then they don’t fit the thread. That they appear in the OP complicates this. Are the Happy Potter books means to be used as a metric for “good?” If so the bar is set pretty low. I enjoyed them when they came out as a bit of low-effort recreational reading, but claiming that they are “good” beyond that level is a stretch unless one’s bookshelf is full of nothing else but “airport fiction.”
To contribute: Caffeine.
The Beatles
Coca Cola
Heinz Ketchup
Sex
U2 (:))
District 9
One Direction
A handful of very vocal detractors would have you believe that it’s the most hated movie of modern times, but its popularity was richly deserved, and it holds up as a great film to this day.
Titanic
The ones that come to mind are the “Game of Thrones” tv show and books. Both really are as good as people say.
I read a book and can’t remember the author - I want to say P.J. O’Rourke or David Foster Wallace - wherein he left his ivory tower and immersed himself in the pleasures pursued by the lower classes. He listened to popular music and took a trip to Branson, Missouri and went to every show he could fit in. Purpose of the book was to prove how ‘bad’ popular culture is, but he found out that it wasn’t really ‘bad’, just…mediocre. Not good, not bad, but somewhere in between.
I think the assessment of the Harry Potter books by Beastly Rotter and JLRogers is quite good. The first few read like Hardy Boys mysteries.
What is good:
To Kill a Mockingbird
Lord of the Flies
The Outsiders
and the Harry Potter movies. All of them are decent, but the third, Harry Potter and the Prisoner of Azkaban, stands out.
Once you get above a certain level of competence, “really good” is subjective. Some people hate Harry Potter, some people have rearranged their lives around the Potterverse. Ditto Titanic, Lord of the Rings, Doctor Who, Star Trek, Star Wars, Game of Thrones, Seinfeld, and thousands of other things I can name. Everything I claim is “really good,” there’ll be somebody else out there to point out that it’s crap.
So what? That’s not what the thread is about. It’s opinion.
Oh, and I second the iPad.
It depresses me just how many of the suggestions in this thread I not only disagree with, but disagree with STRONGLY.
I will second chocolate to keep things positive.
The Simpsons during its peak years in the '90s.
Breathing. If you doubt how good breathing feels, try not breathing for a while! 
Saying, “God bless you always! :)” Come on you know you miss it.
Raindrops on roses and whiskers on kittens.
Another vote for The Beatles. Also, the Sopranos and The Godfather movies.
Videos/pictures of cute, baby mammals. AHHH…Why can’t I look away!!
The iPhone is worth all the attention it gets: Well designed, easy to use, and good ergonomics.
Adidas sneakers, art deco, Barcelona, margaritas, the Saturday Night Fever soundtrack, southern BBQ