Personaly, I enjoyed the Matrix series, as well as T3. Now the second and third movies in the Matrix weren't fantastic, they were not bad. I think the death in production problems they had caused part of the problem, as they had to refilm and rush off to fix things to get the movie out there (Alieah or however you spell it (The evil queen in Queen of the Damned) was supposed to play the part Jadda Pinket Smith did, and the Oracle died mid-production as well.) And T3 was a fun movie, and apparently got James Cameron's approval as what he would have done with it as far as plot goes if he had done one.
Movies that should have sequals:
History of the World Part I
Flash Gordon (Preferably recast)
The Princes Bride (Sequal, Prequal, or even another movie in the same world/style)
2010 (I want to see 2063 and potentialy 3001 personaly)
Starsky & Hutch (Though I'd understand if that didn't happen)
Movies that didn't need sequals:
Speed
Universal Soldier (Though it's debatable if the first one should have been made, but there are 4, including one made with an entirely different cast)
The Crow
Most Single Star slasher movies (Friday the 13th, Nightmare on Elmstreet, Halloween, it gets old)
Highlander (I don't even acknowledge the series, though that's more on a personal disagreement with the star of that show that I won't get into here)
I'm sure I'll think of more.
BTW, Coolest Sequal title: Breakin' 2: Electric Boogalo. Given us comedy ammo for years.