I would say "a good job" is __you would do even if you didn't get paid for it. 空格用one还是that?

one 好一点吧

我也觉得应该用what 因为这句话很容易就会以为是 以a good job 作为先行词 that 作连接词的定于从句 。但是要注意a good job" is 中的is，这里be 动词is要跟的其实是一句表语从句，所以用what

都不好，应该用what

that，因为后面是一句从句

可以用That做成表语从句,不能用One,但可以用The one特指前面提到的A good job

that