I believe the He-177 was a flawed design from day-one, the He-277 a continuation of the flawed dream.....
Some time ago, I read Capt. Eric Brown's book on Luftwaffe aircraft, and he was scathing in his assertion of firstly it's handling, let alone it's projected capabilities....[While there are some who may pour scorn on Capt. Brown's writings, he was THERE during the War, and he did actually FLY them and I feel his experiences give him full justification to write what he did in his unbiased manner....]
The He-177 was really an after-thought, because as stated by others, the German technique was Stukas and twin-engined 'Blitzkreig' bombing; they had no vision of 'long-range heavy bombing', and only started to dabble with it once things started to go awry in Russia......and couple that with the fact they were getting hammered day night by the USAAF and Bomber Command, it was really a 'retalitory idea' in that light....
Germany had lots of 'bright-ideas', and bombing the US with uniquely-special bombers, supported by fighters from aircraft-carriers they never succeeded in building was desperate pipe-dreams.....The US was bombed, albeit very lightly, by the insidious Japanese 'jetstream-balloons', which could have become a real problem, but only a few reached that far....
Through 1943 into 1944, Germany was already doomed by Allied bombing, and if we're talking 'Best Bombers', I found this comment interesting, from Gordon Musgrove's book, ''Pathfinder Force''.....
''With the advent of the P-51D Merlin Mustang the USAAF was able to operate deep into Germany with fighter cover. By attacking aircraft factories it compelled the German Air Force to defend itself and, by it's victories, broke the back of the Luftwaffe [not numerically; in quality and experience.] Shortly after D-Day Bomber Command turned to daylight attacks over Northern France and later, when the British fighters were based on the Continent, they accompanied the bombers to targets in Germany. British bombers were the most economical: they carried a greater load and, by dropping larger bombs, they caused even more devastation. Now, because the force could bomb with precision by day or night or in conditions of 10/10ths cloud, Bomber Command became the most versatile...''
This again highlights the Lancaster and Mosquito in particular, as in these latter stages, these are what aircraft Bomber Command were using more exclusively, plus Halifax's; - the Stirlings and Wellingtons were not by then used in great numerical strength.
From my reading, what Germany WAS then using to good advantage, was the Ju-88, as a Nightfighter, and they were still very lethal....more so than the Bf- 109/110/210/410 and Fw-190, as their speed and firepower [including the 'Schrage Muzik'] and with twin-engined ability, appear to have admirably defended the Reich as best they could...The single-engined fighters seemed more dangerous in the day raids overall.....
For Germany, it was the Ju-88....
For Britain, it was the Lancaster Mosquito.....
For the US, it was the B-17 B-24.....
Over Japan, that was the B-29's ballpark.......