Chamberlain, and most historical premiers, whether you think they made the right choices or not were fundamentally serious people making seriously considered judgements.
Johnson was a liar and a charlatan, motivated purely by ego and narcissism, with lashings of grift on the side. He trashed not merely his government, and Britain's place in the world, but also the entire concept of public service.
He finally left after nearly 60 of his own ministers resigned as they were no longer prepared to lie on his behalf to defend the knowing promotion of a sex pest.
There's an excellent case to be made that he is the very worst prime minister since the post was conceived, and is certainly the worst that I am aware of by a country mile, though I'm no history student.
Truss was a hapless, incompetent ideologue. Had Johnson not trashed the Tory party as well as the government, she would have been unliklely to be in a position to make the foolish choices she did. There's a decent case to be made that she's the second worst ever, merely by dint of her rapid defenestration, but her legacy is far less toxic than Johnson's and she did much less damage.