IMO russia has the best bombers are a previous poster stated however...
The best bomber in the game (non historical battles) is the H6K4, now lets list off why;
Its fast for a boat plane
It can take a huge amount of damage and due to the match making it gets most of the stuff shooting at it will be using crappy guns
it can shrug off rams, while it shouldn't be relied on to do so it can do it in a pinch
it has an amazing armament, the 20mm cannon can 2 hit kill reserve planes, and shred everything else
because its level 3 most of the players you get matched with wont be that good and will most likely fly right behind you, allowing you to kill them with ease with the 20mm and the 2 other mg's (I don't remember the caliber)
because its a boat plane the bottom is extra reinforced making crash landings easier
its bombload while not as good as the Catalina's is still respectable (2x1000 kg bombs at level 3 is far from bad) and can carry good torpedoes
it can fly while it looks like swiss cheese, I've flown mine with the whole plane black except for 2 engines
now that all the good is out of the way, I must list the bad (what little of it there is)
its useless in historical because you cant reload the bombs because its a float plane (that I know of)
it lacks a waist turret, but this is sort of made up for with the 20mm's epic gun depression
the 20mm gunner is exposed and needs to have vitality leveled up to survive a prolonged assault
thats all that I can think of atm!