Series with biggest leap in quality from one entry to the next?

" Everything before Resident Evil 4 -> Resident Evil 4. "

There are so many especially considering how many series revivals there are.

"Street Fighter > Street Fighter 2 "

Killzone   -> Killzone 2

" Super Mario 2 to 3. "

" @MysteriousBob said:

"Street Fighter > Street Fighter 2 "

Does that really count?  >_> "

" @GetEveryone said:

" Everything before Resident Evil 4 -> Resident Evil 4. "

Either this or Zelda II: The Adventure of Link -> The Legend of Zelda: A Link to the Past. "

I agree with the guy that said Metal Gear 2 to Metal Gear Solid

I think Metal Gear Series takes everything to a new level every time. MGS1 was superior to the metal gear games, MGS2 improved(gameplay wise) over MGS1, and MGS3 was a whole different breed of awesome compared to 2. While some would disagree, I think MGS4 ushered the MGS franchise into the modern era perfectly. 

" Dragon Age 2 -> Dragon Age: Origins "

Wolfenstein 3D -> Return to Castle Wolfenstein.  Staggering jump in wow.  I feel the reverse with Doom 3 as Doom 1 and 2 are still loads more fun to play and replay.

Systerm Shock 2-Bioshock.

" @MysteriousBob said:

"Street Fighter > Street Fighter 2 "

Does that really count?  >_> "

GTA 2 --> GTA 3. Talk about a quantum leap!

ME1 ---> ME2

GTA2 - 3, AC - AC2

" DMC2 -> DMC3  Fuck you DMC2. "

" How the hell no one mention Battlefield 2 -> Battlefield 3 yet? "

Guitar Hero 2 -> Rock Band

" Jak n' Daxter ---> Jak II "

Just Cause to Just Cause 2.

FFX -> FFXII

Going from C&C: Red Alert 3 to C&C 4: Tiberium Twilight was pretty rough for me.