Counterpoint to my other thread about bad video game sequels - what are some of the BEST sequels you've ever played? What made them better than the originals?
Battletanx Global Assault was a nice sequel to the original Battletanx. They took all of the complaints about the previous game out (Such as the op Goliath tanks with their Hp allowing them to take/use nukes at point blank range) and kept everything that everyone liked about the previous game (The gameplay and the different types of tanks). This is how one makes a sequel, imo.
Spyro the Dragon was great. Ripto's Rage/Gateway to Glimmer built on the original very nicely. But Year of the Dragon put it over the top.
The first one that comes to mind is The Legend of Zelda: Majora's Mask. It refined the Ocarina of Time mechanics while adding new and interesting elements to it. Adding to that an interesting storyline and an Eldritch Abomination of a villain that had nothing to do with the King of Evil, and you have a game that hasn't been beat in 19 years. (And yes, I love all of the other Legend of Zelda games, with the possible exception of Phantom Hourglass, Spirit Tracks, and Skyward Sword - possible because I haven't played the latter and only played some of the former two, but do recall a lot of the issues that others have had with them.) Another is Mass Effect 2. Now, Mass Effect was already a great game (though Mark could have done a bit better job delivering his lines, but that's being nitpicky), but while I am sad they went with a limited-ammo setup and took the large variety of weapons that were in the first one, the combat is more streamlined. The story is really interesting (it's not very often that the hero dies in the first five minutes of a game), the characters are (arguably) more interesting (though Ash is still my favourite), and the planets are a lot more interesting to visit (though I miss the Mako).
Binding of Isaac: Rebirth was a nice upgrade from BoI: Wrath of the Lamb. Pixellated sprites instead of highly cartoon-y ones, much smoother shooting and movement controls, and more items, areas and characters. Easier for you, but the updated enemy AI and increased enemy types make it more balanced as well. Even without Afterbirth and Afterbirth+ it makes for a good game.