Ki-67 has rather light load for most work. But its speed makes up for a lot of it. Much more survivable IMO than the ju-88 in the main.
Ju-88 has better load, is more flexible, (dive brakes) but its defensive guns bite. Like anything else depends on what your trying to accomplish and how badly you want to land.
Ju-87 = great vs vehicles.
D3a1 val = Well I love the val, what can I say.

It may be worthless as a bomber, but it is a wonderful trainer Aircraft.
Its a joy to fly just for flying sake. It will turn with any plane in the game, yes even the A6m2 if you use flaps. It has popguns, but they have a near endless supply of ammo for them.
The real joy of flying the val in combat is suckering other bigger, faster, better armed planes into turning with you. Ping them once or twice, then fly them into a hill or sucker them into a split S so they lawndart.
There is no feeling in the world like landing 2 kills in a val vs late war aircraft.