Apples and oranges; medium bomber; B-26, most versatile (of those three); Ju 88, low level strike; B-25. If I was to choose a 'Best Medium Bomber of the War' however, it'd be the Mosquito.
It's a tough call, I echo the sentiments of others here, all three were excellent aircraft in their own right and had sterling combat records, although the claim that the Ju 88 was a failure in the BoB is not strictly true; the Ju 88 performed the roles asked of it very well - it proved a good bomber and anti-shipping aircraft, it's biggest failure was in common with all other German bombers at that time in its inadequate defensive armament. During the battle, the Ju 88 proved to be the hardest German bomber to shoot down; once it's load had been dropped, it frequently demonstrated why the Germans called it the schnellbomber.