Most improved sequels

[QUAKECore89]

-Wind Waker (it has combo).
-Half-Life 2 (physics).
-San Andreas (bigger map).
-Halo 2 (more polygons).
-Metal Gear Solid 2 (improved cinematic).
-Doom 3 (improved graphically).
-Metroid Prime 2.
-Pokemon Gold/Silver (two freaking regions)

Edit: And of course, Super Smash Bros. Melee (Faster paced)