Which Canon PowerShot digital camera is the best?

There is no best, but...
I own the a620, and love it for the higher megapixels.
but the a610 is less megapixel but has less noise in the picture.

Really depends what you are looking for.

Personally for me, if i was on a budget, and still wanted a good camera I'd go for the a610, the price is awesome and its a great camera. I could post many reviews on all the Powershots, but I am sure you can research them yourself easily enough.

Canon Powershot S2 IS