I just saw some news about Halo 2 turning 20 years old, which reminded me of the hype and excitement around that game when it came out. For me, this is still the biggest advancement in a sequel that I can remember. We were already playing the first Halo heavily at the time, but when 2 came out.. Dual-wielding, hijacking vehicles, usable energy swords, driveable wraith tanks.. and the battle rifle! As a young gamer easily fascinated by such things, the long list of new features was simply unreal to me at the time.

We were regularly playing four player split-screen deathmatch and playing the campaign in co-op. In retrospect, we have probably returned more to the first game and 3 when playing co-op on the legendary difficulty (because of the jackal snipers in 2), but it's one I find myself returning the most to when playing Halo single player. And the remaster was the reason I initially bought a Xbox One back in 2014.

What games do you remember that stood out with having a big list of improvements over previous iterations? Some honorable mentions for me would be Fable 2 and GTA: San Andreas.

I'm always absolutely amazed that Warcraft II was released only a year after the first one. Just the difference in graphics is astounding, and the whole game is so much more polished it makes a world of difference. In comparison Warcraft 1 is still very primitive, and W2 feels pretty much entirely modern. It's like going from a bow & arrow to a colt revolver in one go.

I guess Duke Nukem 3D is a tremendous leap, as it is a direct sequel to the side-scroller DN2. It's just that I never played the platformer Duke games, so I didn't experience that first hand (and other early FPS games released before DN3D kind of mitigate the "wow factor" of going from the 2D Duke to FPS Duke).

The Witcher 2 from 1.

That's less a leap and more of a step sideways and on a rake.

I was actually very disappointed with 4 when it came out. I guess I didn't enjoy the more realistic gameplay and darker setting as much. I've returned to it years later, but I have still never finished it. 5 was amazing though, and they always genuinely put a lot of work into these games. The worlds just keep getting bigger, and they don't lack in content. 6 will surely deliver.

Whether you consider them "improvements" or not is subjective, but at least there are very massive differences between Leather Goddesses of Phobos and Leather Goddesses of Phobos! 2: Gas Pump Girls Meet the Pulsating Inconvenience from Planet X.

Very few sequels have had their entire interface and looks changed, but still basically remaining faithful to the original idea. Leather Goddesses series can probably be counted successful in doing that.

Going from GTA II to GTA III. SPecially considering it is only 2 years between the two games, and it is a complete genre shift as well as establishing a new genre.

Could also add the Fallout games (depending on how you count Fallout 2 or Brotherhoos of Steel the last part) into Fallout 3

Dragon Warrior 2. DW1 is a rather simple game, with just one character in the party, one enemy, and some very primitive mechanics (though still conventional-feeling, unlike early Ultima). But then you get DW2, which gives you a 3 character party, has you fighting multiple enemies, not to mention a much bigger world with even the addition of a ship to travel the world.

DW3 further added character creation and a class system, so you get to choose your party composition. (Also 4 character parties, as well).

Final Fantasy 5 is a significant improvement over previous entries. Not only can you choose each character's class (a feature found in FF1 and FF3 that's missing in FF4), but you also get to choose a secondary ability from a class you've been in before, creating so many possibilities, far more than in any earlier FF game. There's also so many interesting new abilities, like Blue Magic (which the game gives you early), Sing (Bards are actually useful, for the first time in the series!), Mix, and many other less prominent ones. It really is a shame that FF6 was a step backwards, no longer allowing class selection, and getting rid of some of the more interesting abilities (I'd rather they kept Mix rather than Throw).

I could also mention Castlevania 2, though that game felt like a different platformer sub-genre than either 1 or 3 (and one I wish there were more of).

Would have to mention Super Mario 64. That game was such a massive leap in the Mario franchise and platform games in general it is essentially the gold standard and one if the few games to successfully transition from 2D to 3D.

For me would be Elite 2 Frontier compared to the original Elite you could land on planets, buy new ships, bigger universe and all round much better gameplay.

Metal Gear 2 (SNES) to Metal Gear Solid (PS1)
Metal Gear Solid (PS1) to Metal Gear Solid 2: Sons of Liberty (PS2)

Those two where absurdly impressive in my opinion.
When Metal Gear Solid 2 trailer dropped it caused something from another world in players minds.

Sorry for being that guy but Metal Gear 2 was never officially released on the SNES, it was an MSX2 game.

No problem. :)
Thanks for the correction!

And Snake's Revenge was a NES game.