As far as picking the ''Best Medium Bomber'' goes, it's abit like cricket etc...it's about how many ''runs''....
Initially I agree with wmaxt, that both the Mossie B.25 were excellent in this role, especially in terms of length of service.....and Nighthawk also has pointed-out that the Ju-88 was also an exceptional Medium bomber, which was indeed the case, credit due....
I think the A-26 was an excellent aircraft...still is...but one point here is that it didn't actually come on line until 1944, when it was intially used by OSS to drop agents into Europe...and then was further employed in it's various roles....
As stated, it has been universally accepted that the B-25 was ''The Best Medium Bomber of WWII '', as it was in there real early, used by both the RAF and US in virtually all theatres, and aircraft such as the B-26 and A-26 were supposed to be replacements for the B-25.....It earned the honour also by 'Doolittle's Raid', which was of enormous boost for the American morale at the time, as the first blow, back at Japan...not unlike RAF's ''1,000 Bomber Raid'' against Cologne.....
We cannot discount the Mosquito's contribution, the ''Light Night Strike Force's'' raids into the heart of Germany, UNARMED and ''cookie-capable'', they were counted as 'heavy-raids,' by the Germans themselves....Over and above everything else Mossies did in their various roles, also the fact that they were only two-crewed, where every other bomber was multi-crewed...Mossies were 'economy-personified' in a time of many shortages...
Anyway, my predisposition to the Mosquito really goes without saying, but I believe that what the B-25 was to America, at the start of the War for them, the Wellington was for Britain, followed then by the Mosquito, and the US went on with the B and A-26's......
Germany's Ju-88 was their best, and maybe the Cant as CC says, for Italy, although the SM-79 was probably used more, and Russia had it's Pe-2 and Il-2, then Japan's best medium, I feel, was probably the Ki-46......
Hard to pick one, really....but I believe the B-25 got it years ago....which is hard for me, because I named my son 'Mitchell', and I cherish the Mossie Mitchell both.............