Aliens
The Empire Strikes Back
Harry Potter and the Prisoner of Azkaban
In each case because these movies were the only ones in their series I liked enough that I thought they were worth owning.
Oh, hell, yes.
Oh, that reminded me. I also have Prisoner of Azkaban and Goblet of Fire, but not any of the other Harry Potter movies. I love, love, love the books, but the movies were largely disappointing. Prisoner of Azkaban is by FAR the best of the movies.
I have several of the Phantasm movies on DVD, but not the original. Still searching for a cheap used copy.
PS - but I do have all of the Evil Dead movies.
Army of Darkness. Also The Good, the Bad and the Ugly. Finally, while not currently in my collection, I’d totally pick up Mad Max 2: The Road Warrior sometime if it was cheap. No interest in the first one. MM2 is as awesome as MM1 isn’t.
I do, but I kid you not, I’ve never put it in and watched it. It just kind of sits there.
I think I have Drunken Master 2 as well, but I am not sure if I own Drunken Master 1. I have seen it, though.
Another Xmen 2 here. (It’s really superior to X1 or X3).
Also Chronicles of Riddick (haven’t watched it yet, picked it up cheap)
Technically speaking, Legally Blonde 2 (though it’s on my “to sell” pile after one watch and can’t get out of here fast enough.)
Not sure if this counts, but I have Jay & Silent Bob Strike Back, Chasing Amy, and Clerks, but not Mallrats or Dogma.
The Road Warrior, but not Mad Max.
I think Iron Man 2 was a much, much better movie than the first one, so it’s the one I own. Diito for Addams Family Values.
Star Trek 2: Wrath of Khan, 6: Undiscovered Country, First Contact.
Army of Darkness
The Chronicles of Riddick
I have II. I’d hesitate to take TMP even as a gift.
Spider-Man 2, which I received as a gift. I bought Evil Dead 2 and Army of Darkness, but don’t have the first Evil Dead movie.
Dark Knighr. My brother owns the first one, and we got him the second one for Christmas the year it came out, but he had already bought it for himself. We could have returned it, but I loved it, too.
I own a digital copy of Boondock Saints II. I rented the first one and had a hankering to watch the second, but it wasn’t available for rent, only purchase, and I thought, why not?
I own Christmas Vacation, but not Vacation.
The Dark Knight, but not Batman Begins
also, The Color of Money, but not** The Hustler**
Star Trek II
My Girl 2
Meet the Fokkers
For a long time, I owned Bridget Jones: The Edge of Reason, which I love, but not the original movie, which I didn’t really like. I think I finally picked up a copy of Bridget Jones’ Diary cheap from a rental place that was going out of business.
Harry Potter 3
Batman Forever and Batman & Robin. I’ve got the Nolan trilogy too, but have no desire for the Tim Burton movies.
Clerks was the first in the series, so I’d say it doesn’t count. I’d also say that Dogma and Mallrats are two of Smith’s three best films, so you’re overdue for a trip to the store. 
Babe: Pig in the City. But not Babe, which I should really pick up somewhere.
That’s the only one I can think of. I have all the Star Trek movies, including TMP and the J.J. Abrams reboot, except Nemesis. I have all the original Alien trilogy, four Terminator movies, and several other series but no other orphaned sequels that I can think of.
Karate Kid II.