Which is the best digital camera from canon in POWER SHOT 5 Megapixel category?

the PowerShot S2 although it is more expensive.

Well, if you mean you are looking at only Canon PowerShot cameras with 5 Megapixels then it seems the PowerShot S2 is your best bet. However, there are cheaper 5 Megapixel cameras and it really comes down to what you are looking for.