Maybe yes, maybe no. I think that more people are familiar with Im-Ho-Tep (partly because of the use of the name more recent updates) and the search of Im-Ho-Tep for his reincarnated beloved (although that was duplicated in the 1940s sequels; it was also retold in the recent remakes) than with Kharis as the mummy’s name, or of his animation with Tana Leaves than with the Book of Thoth.
Ultima II was a great game for its time, though it hasn’t aged as well as the others. Single character, no zoom in tactical to fight as a party. There was also an alien spaceship and space travel, which for some mysterious reason never made the cut for inclusion in subsequent Ultimas.
That being said, I never played the original Ultima, so Ultima II would still count in my book.
I don’t think the film Angels & Demons ever addressed when it took place in relation to DaVinci Code. I imagine there’s plenty of people who have only seen the movies who think Angels & Demons is the sequel.
I’d say The Road Warrior almost qualifies as a remake of Mad Max rather than a sequel. Desperado the same for El Mariachi.
I guess the least well known of the Wallace and Gromit movies is the first, A Grand Day Out.
Aside:
j
World War II
Fun trivia/bar-bet fodder: Who starred in Rambo II? Answer: no one. There never was a Rambo II.
I think I can top that one: In the House horror movie franchise there was a I, II and IV, but no III.
Huh? How do you figure? Road Warrior is what happened to Max after the first movie. Plots totally different.
Bikers destroy his life, he gets Mad, ends up roaming the waste-land. Cue the second movie.
And before Ultima, there was Alkabeth.
Ultima and Ultima II aren’t really all that different. Same basic gameplay mechanics, same basic plot, but different settings. ETA: “settings” as in the world the game took place.
I’d have to agree that Ultima III was when the series really began to be known, though. It was the first one I played, at the time of its release. I’d seen a few friends playing the first two, and decided I wanted my own copies, and had a hell of a time finding them. Ultima III was always on the shelves, though.
What about Disney’s The Rescuers Down Under? Maybe it just seems that way to me because the first movie came out before I was born, while the sequel came out when I was a kid. I hadn’t even heard of the original The Rescuers or realized The Rescuers Down Under was a sequel to anything until I was an adult. But I don’t know if that’s true for the general public, or if it’s just me, of perhaps people in my general age group.
Maybe they were afraid of the “odd numbers suck” meme. 
The sequels to the original Friday the 13 are probably better known than the original movie. If only because:
In the original movie Jason’s mother is the killer not Jason himself.
I’d wager that Star Trek II: The Wrath of Khan is better known than Star Trek: The Motion Picture—not in the sense of people not knowing of the existence of the earlier movie, but in the sense of a lot fewer people knowing anything about the movie or what happens in it.
Because of the Disney film, the Black Cauldron is the second and best known Prydain book. The movie really mushes together the first two books.
I’d argue Ultima IV is the most iconic.
The cartoon image of a dessicated guy lurching around wrapped in bandages is from the Tyler/Chaney movies. Karloff was quite spry after he was revived, and had no trouble passing for a (relatively) normal human.
In each case, the second movie contains flashbacks to the first. In each case, the second movie makes a point of mentioning that the hero is handicapped from an injury he sustained in the first.
The third movie was released in Australia as House III, but in the U.S. as The Horror Show. The second and third movies had no onscreen connection to the first, just some production crew in common, and the “strange things happen in this spooky old house” motif.
Since The Wrath of Khan got so many reviews that were better than Star Trek: The Motion Picture’s, I wouldn’t be surprised if more people outside Trek fandom have seen the former without ever seeing the latter.
I surprised nobody has mentioned Bill Cosby’s Leonard Part 6. I don’t know anybody who’s seen parts one through five.
Though it’s a well-known film, I’d reckon that the more current Bond films are better known than Dr. No - or at the very least, it is not the best-known Bond film.
Batman Begins is not an unknown movie but The Dark Knight is much better known.