I would say from kh1 to kh2 was a good improvement

Yeah I meant over Jak 2 :P

Most improved:

F-Zero X -> F-Zero GX
Halo 2 -> Halo 3
Oblivion -> Skyrim
Super Mario Kart -> Mario Kart 64
Mario Party -> Mario Party 2


Sequels that are not necessarily better, but excellent in their own rights:

Ocarina of Time -> Majora's Mask
Goldeneye -> Perfect Dark
Perfect Dark -> Perfect Dark Zero
Halo 3 -> Halo 4
Banjo-Kazooie -> Banjo-Tooie
Super Mario 64 -> Super Mario Sunshine

Uncharted 2
Assassin's creed 2

Dissidia -> Duodecim

Original was pretty good, but Duodecim is a masterpiece in my eyes

GTA III
God of War II
Assassin's Creed II
Killzone 3
Uncharted 2
Batman: Arkham City

For me...

Red Steel 2: The original is nigh unplayable, but the sequel plays like a dream thanks to motion plus.

Mario Galaxy: One of the best games ever made in my opinion, an enormous leap over the mediocre Sunshine.

Killzone 3: Didn't like the first two games, but the more responsive controls, greater variety, and the option of playing with Move made KZ3 quite fun.

Resident Evil 4: THE best game I've ever played. By contrast, I found the fixed camera angles made RE1-3 clunky and awkward.

Agreed. Dead Space 1 is one of my favourite games of last gen, but I found 2 and 3 to be meh.

Street Fighter 2, Sonic 3 & Knuckles, Paper Mario TTYD, Super Mario Bros 3, Mass Effect 2, Mega Man 2, Uncharted 2 to name a few.

You don't have very good memory guys, or too young :)

1) Virtua fighter 2 ! By a long margin, better graphics and gameplay over Virtua Fighter, on the same Platform, Sega Saturn ;) Don't take sequels developed on the next hardware, it's obvious you have monster improvements, like MGS2 >>> MGS.

2) Far Cry 3 >>> Far Cry 2 on XBox 360.

3) Uncharted2 was also a big improvement.