What do you use it for? I usually try to buy the shortest and lightest scope that still fills all my requirements.
Will you use a mildot reticle? If so, will you use magnification past the "true mildot" mag of 20x?
To me the 8.5-25x42 only makes sense if you want to be able to use that bit extra magnification to take a peek at your target. Otherwise go with the 6.5-20 for a little less weight and length.
The 8-32x56 is ideal for range finding with your scope at the very high 32x magnification, used for Field Target and/or very precise target shooting. The large 56 mm front bell will capture more light than a 42 mm which will compensate a bit for the image darkening as you crank up the magnification.
This scope will be much heavier than the other two, especially front-heavy because the large front bell hangs out far ahead of the rifle's trigger.
For hunting, plinking and all-around convenience I would choose the 6.5-20x42, for target shooting the 8-32x56.