B26s, fast and a small target. They used to be at the bottom of this list, for me, because all they used to need is one burst of cannon at the wingtips. But since then the hit bubble granularity has improved and it's nowhere near as easy as it was then.
I used to be able to consistently barrel roll a 262 around a formation with one burst per roll killing either of one bomber's wingtips, effectively killing a formation in about 30 seconds, but not anymore!
Lancs are easy, no belly guns and a big target surface means you just come in from under as fast as possible, land your hits and pull off on the outwards half of your pass in an unpredictable way (for the gunners).
Pretty easy.
24s do catch fire pretty easily. Otherwise they'd be tougher than my personal second-hardest bomber -
B17s are slower than 26s but otherwise the best defended bombers. I've always hated having to go against their guns.
67s are made of paper despite being fast and well defended, 88s are undefended, 25Cs are slow and don't even have a belly gun, 234s are undefended provided you have time to setup a pass (just land some hits on those vulnerable turbines)... The only others worth a mention are the Il2 for its toughness, and the 25H for its ability to pull a counter attack with its 8 nose 50's and the cannon if you're unlucky or neglectful enough to fly predictably enough for it to hit you on the outward part of your pass.
