I'd just like to thank kingofwale and Soriku for starting this awesome thread.
Here are some trends we can look at:
Animal Crossing on N64: 0.26 million
Animal Crossing on GC: 2.98 million
Animal Crossing on DS: 9.47 million
God of War on PS2: 3.20 million
God of War 2 on PS2: 2.11 million
God of War on PSP: 0.75 million
As you can clearly see from these numbers, one franchise is growing exponentially, and one is shrinking very quickly.