Assuming we're not counting obvious ones (Tetris is a shoe-in), then I'll go with The Sims. I lost track of all the expansion packs a long time ago. Seriously, I stopped keeping track after The Sims: Hot Date. How many more have there been after that one?

The Sims 3

Core game

• The Sims 3

Expansion packs

• The Sims 3: World Adventures - Upcoming

The Sims 2

Core game

• The Sims 2
• The Sims 2 Special DVD Edition

Expansion packs

• The Sims 2: University
• The Sims 2: Nightlife
• The Sims 2: Open For Business
• The Sims 2: Pets
• The Sims 2: Seasons
• The Sims 2: Bon Voyage (The Sims 2: World Adventure in some regions)
• The Sims 2: FreeTime
• The Sims 2: Apartment Life

Stuff packs

• The Sims 2: Holiday Party Pack (The Sims 2: Christmas Party Pack in the UK and Ireland)
• The Sims 2: Family Fun Stuff
• The Sims 2: Glamour Life Stuff
• The Sims 2: Happy Holiday Stuff (The Sims 2: Festive Holiday Stuff in the UK and Ireland)
• The Sims 2: Teen Style Stuff
• The Sims 2: Celebration! Stuff
• The Sims 2: H&M Fashion Stuff
• The Sims 2: Kitchen And Bath Interior Design Stuff
• The Sims 2: IKEA Home Stuff
• The Sims 2: Mansion & Garden Stuff

Compilation packs

• The Sims 2: Holiday Edition (2005) (The Sims 2: Christmas Edition in the UK and Ireland)
• The Sims 2: Holiday Edition (2006) (The Sims 2: Festive Edition in the UK and Ireland)
• The Sims 2: Pets Edition
• The Sims 2: Seasons Edition
• The Sims 2: Limited Edition
• The Sims 2: Deluxe
• The Sims 2: Double Deluxe
• The Sims 2: University Life Collection

The Sims Stories

• The Sims Life Stories
• The Sims Pet Stories
• The Sims Castaway Stories

Console editions

• The Sims 2 (GameCube, PlayStation 2, Xbox, Game Boy Advance, Nintendo DS, PlayStation Portable, Mobile Phone)
• The Sims 2: Pets (Wii, GameCube, PlayStation 2, Xbox, Game Boy Advance, Nintendo DS, PlayStation Portable)
• The Sims 2: Castaway (Wii, PlayStation 2, PlayStation Portable, Nintendo DS)
• The Sims 2: Apartment Pets (Nintendo DS)

The Sims

Core game

• The Sims

Expansion packs

• The Sims: Livin' Large (The Sims: Livin' It Up in the UK and Ireland)
• The Sims: House Party
• The Sims: Hot Date
• The Sims: Vacation (The Sims: On Holiday in the UK, Ireland and China)
• The Sims: Unleashed
• The Sims: Superstar
• The Sims: Makin' Magic

Compilation packs

• The Sims Collector's Edition
• The Sims Party Pack
• The Sims Deluxe Edition
• The Sims Double Deluxe
• The Sims Mega Deluxe
• The Sims Triple Deluxe (UK and Ireland only)
• The Sims Complete Collection (The Sims: Full House in Australia and New Zealand)
• The Sims Expansion Collection
• The Sims Expansion Three-Pack

Console editions

• The Sims (PlayStation 2, GameCube, Xbox,)
• The Sims Bustin' Out (PlayStation 2, GameCube, Xbox, N-Gage, Game Boy Advance)
• The Urbz: Sims in the City (GameCube, PlayStation 2, Xbox, Game Boy Advance, Nintendo DS)

Other

• The Sims Online

Mario.

I find the Guitar hero list, scarier than the FF list.... i mean... GH has been around for what?? 4 maybe 5 years?

Mario ? Star Wars?

the most milked franchise ever is Mario. theres over 150 games of mario, most of which is the same point/ things to do.

second would be final fantasy

third would be guitar hero (my opinion)

mario

Mario is an IP, not a franchise.  They have different franchises that use the Mario IP, like Super Mario, Mario Kart, Mario Golf, etc.

Guitar Hero, all that games are released in a few short years.

Someone get a list of the Megaman games up here. That may actually rival FF for how much it's been milked. Not that I actually have much of a problem with that, being a fan of the mainline Megaman metaseries, but it's tough to deny that Capcom could run its own dairy off of those games.