The Boeing B-17 Flying Fortress, or the Consolidated B-24 Liberator?

Neither one was exceptionally well suited for the European war...by the end of the war, we had a lot of modifications on the table and a new plane underway

Both. Each had its features. The B17 seemed to work out as well as expected in Europe using the large formations of the time with interlocking fire BUT smaller bomb loads. The B24 was the best choice in the Pacific due to its longer range and larger bomb load. It performed well till the end of the war but was shadowed when the B29 arrived in the PTO.

They are both great weapons platforms. The B-17 is the iconic symbol of the air war over Europe. I love them both.