I read all the posts in this thread and we need to define how to chose "the most milked franchise".
Some could say that Mario and Zelda are the most milked one, but those games at least are almost always GREAT games. They always some different art style, mechanics, stories....
We have the CoD franchise which had 15 games, but all of them are pretty much the same game. They have little innovation between two titles..
Then there are those Fifa and Sims games. Those are games which have no innovation at all. Excluding the graphic improvement.
My definition of "milking a franchise" would be: "Trying to earn as much as possible from the franchise with as little as possible investments."
And according to my definition, Nintendo's main franchises aren't milked.
Zelda has 3-4 years between two titles. It has always a different story, different art style, some different mechanics, some innovation...
Mario is almost always a completely different game from the previous one (excluding some portable games). Galaxy and Galaxy 2 are "same" jet again a lot of fun. Other Mario games are usually completely different.
CoD was good up to the 4th title. After that it goes downwards. After the 4th title, that's called milking.
Fifa, Sims, Madan or what ever the name is, those are the most milked franchises. They have 10$ development costs between 2 titles, especially the Sims.
NOTE* Pokemon is NOT a Nintendo franchise. Yes, it's exclusive for Nintendo consoles, but it's Gamefreak who are developing the game, not Nintendo. It's like saying Uncharted is Sony's franchise.