There are multiple threads about sequels that are as good, almost as good, or better than the original film, but I will list a couple here:
Aliens - Although the first film is my personal favorite, most folks seems to like Aliens the best. It is certainly a fantastic film, and regarded as one of the better action sci-fi films ever made by many.
X2 - This one is better in almost every way than the original, IMO.
The Empire Strikes Back - Again, I like the first Star Wars the best, but this one is considered to be the most well directed film of the bunch, and also the more dark and operatic film, as well.
T2 - I notice you skip over comparing T2 to the original, while lambasting the third film (rightfully so, IMO). T2 another example of a general consensus I have noted in that it outshines the original. I disagree, personally, as I like the gritty tech-noir feel of the first, but T2 is clearly well liked, and is considered to be a groundbreaking film. It is certainly well liked.
Star Trek II: The Wrath of Khan - The director's cut of Star Trek:The Motion Picture made it a much better, more concise, and better paced film, but Star Trek II is still much better.
A Shot in the Dark - Not a direct sequel to The Pink Panther, but it is the second Clousou picture, and I think you would be hard pressed tyo find someone that likes the first film more than this one. Superior in almost every way. One of my favorite Sellers films.
The Good, The Bad, and the Ugly - The best of the trilogy, hands down, and it's the third film in the series.
The Road Warrior - I certainly like the second film more than the first.
Bride of Frankenstein - Hard to top the first film, but alas, this does it.
Clear and Present Danger - A loose sequel to, and better film than Patriot Games.
Exorcist III : Legion - This is one of my favorite suspense/horror flicks of all time. I won't say it is better then the original, because it probably isn't, but it is DAMN good. This is a great set of films for this topic, because The Heretic is quite possibly one of the worst films I have ever seen, but it in no way tarnishes the first film, nor the third, IMO.
Evil Dead II : Dead by Dawn - Half sequel, half remake, it is the definitive Evil Dead film.
The Dark Knight - The most recent Batman film is the best of the bunch.
So no - sequels don't ruin the originals in many, many cases, IMO.
I'm not sure that "sequel" is the correct designation for most of the film series mentioned in this thread, i.e. a literary or cenematic work
continuing the course of a story begun in a preceeding one. Take the James Bond series of films for example: All are about a British secret agent and a few other characters reappear from time to time, but there is no sense in any of the films or the books on which they are based of the progression of the same story line. It's all just a haphazard series of incidents. Same is true with
Die Hard series: you've got, what, 3-4 characters who keep bumping into each other (particularly the wife and newsman) but there is nothing in any of the plots that suggest a continuing story. Each one stands alone, with no dependence on the previous or the following film. Same thing with
Lethal Weapon and IMHO, the
Alien series OK, there's a tentative link with the spacewoman keeps being brought back to capitalize on her experience in combating the alien creature, but in each case her motivation is different (survival in one, rescue in another), she gets more or less assitance from the people or robots who are with her, and the alien she's fighting in any one film dies at the end; we think in the next film she's fighting the same monster again because they all look alike to us. Those films are no more sequels than the series of Marx Brothers films or the
Thin Man series with the recurring appearances of Nick and Nora Charles.
The most successful sequel (maybe only one of two
truly successful sequels as far as continued plot and storyline) IMHO was
Godfather II, which is both a prequel and a sequel to the events in the original
Godfather. The storylines entertwine perfectly because all the events in the two films are in the original best-selling book. Moreover, in
GFII you learn more about what motivates the Godfather and what influences the development of his relationship with other mob members and what influences the development of his children (the middle brother between Sonny and Mike is weak because he was a sickly baby). Godfather III on the other hand is like the old Pin the Tail on the Donkey game at a child's party--just blindly stuck onto the original picture in hopes that it will appear to be a part of it.
Yet even
Godfather II appears to me to fail in one important aspect. As I see it, the only true sequel is the one that the writer/director intended to make from the start--where someone says, "I'm going to write (or film) this one segment of the story and then continue it in a second book (or film) immediately afterward." I don't think the Godfather II sequel was planned from the start as were the first three
Starwars or the
Lord of the Rings films. Those two were truly sequels in that sense, but to me
Starwars was less successful than the
Godfather and
LOTR series in that the second of the three films just wasn't as good as the other two in plot and characterization.
The way I see it, most so-called "sequels" are the result of accountants hoping to recreate a box-office success by cloning a film that was perhaps an unexpected hit, so it's "Give me another
Die Hard, another
Terminator, another
Superman, etc., as much as like the original success as possible with the same cast and a similar plot. Such films seldom are anywhere as good as the original, but the original has created a fan base that will pony up admissions for the chance to relive some of the thrills of the original hit.
PS Sedai, I agree that
A Shot in the Dark is in many ways much better than
The Pink Panther, but that's because
Shot was first a successful play in Europe before being made into a movie. As I remember it, Peter Sellers' character Clousou was a totally surprise hit in
The Pink Panther and the studio wanted immediately to put him and that character in another movie while he was still hot, which is how the Clousou character got grafted into a script in which he did not originally appear. But because
Shot did not turn out to be the giant hit they were hoping for, they decide to combine the Pink Panther title and cartoon character with Sellers and his Clousou into the very successful series of Pink Panther films