What was the main bomber used by allies in World War 2?

This is difficult to answer, because the term 'main' bomber is a little vague. I don't know the exact number for each type of bomber used by Britain and the US, but can offer a personal opinion. The Lancaster was "...the monarch of night bombers in Europe" for Britain. The B-17, B-24, and B-29 were the three heavy bombers most in service for the US. The B-17 in Europe, the B-24 in the Pacific (long range) and B-29 (atomic bomber) were used where most needed. Hope this helps.