B-29 hands down.
Its fast sustainable cruise of 342 mph would make it almost impossible for prop fighters to intercept, especially at 30,000 feet.
The Lancaster is a close 2nd, and carried similar bombloads, but with a top speed of almost 100 mph slower, and a service ceiling almost 10,000 feet lower, it was much more vulnerable. And its round trip range of 2500 miles is about 1000 miles less than that of the B-29A. Finally, the 8 x .303 guns in 3 manned turrets of the Lancaster cannot compare with the 10 x .50's in 4 computer assist remote turrets (using a computing gunsight and fire select system) plus 2 x .50's in the tail. The B-29 had the best defensive armament setup of any bomber in WWII by a large margin.
=S=
Lunatic