Best measure to find difference between 2 images?

hello;

I have 2 images (of same size), both gray levels.  Say A and B.  B is a 
modified version of A, and I'd like to measure how far B is from A.

I currently use RMSE and also use relative error,absolute error and 
histogram difference. But I 'like' RMSE the most so far.

Is there a better method to quantify, in one number, the difference
between 
2 images that I am overlooking ?

thanks,
Nasser