One that gives good reason for someone hearing it who wasn’t previously aware of it to increase their credence in the thing it’s an argument for. (And, since really we should be talking about better and worse arguments rather than good and bad ones, a better argument is one that justifies a bigger increase.)
For instance, consider the arguments about how COVID-19 started infecting humans. “It was probably a leak from the Wuhan Institute of Virology, because you can’t trust the Chinese” is a very bad argument. It makes no contact with anything actually related to COVID-19. “It was probably a natural zoonosis, because blaming things on the Chinese is racist” is an equally bad argument for the same reason. “It was probably a leak from the WIV, because such-and-such features of the COVID-19 genome are easier to explain as consequences of things that commonly happen in virology research labs than as natural occurrences” is a much better argument than either of those, though my non-expert impression is that experts don’t generally find it very convincing. “It was probably a natural zoonosis, because if you look at the pattern of early spread it looks much more like it’s centred on the wet market than like it’s centred on the WIV” is also a much better argument than either of those; I’m not sure what the experts make of it. In the absence of more cooperation from the Chinese authorities (and perhaps even with it) I would not expect any argument to be very convincing, because finding this sort of thing out is really difficult.
One that gives good reason for someone hearing it who wasn’t previously aware of it to increase their credence in the thing it’s an argument for. (And, since really we should be talking about better and worse arguments rather than good and bad ones, a better argument is one that justifies a bigger increase.)
For instance, consider the arguments about how COVID-19 started infecting humans. “It was probably a leak from the Wuhan Institute of Virology, because you can’t trust the Chinese” is a very bad argument. It makes no contact with anything actually related to COVID-19. “It was probably a natural zoonosis, because blaming things on the Chinese is racist” is an equally bad argument for the same reason. “It was probably a leak from the WIV, because such-and-such features of the COVID-19 genome are easier to explain as consequences of things that commonly happen in virology research labs than as natural occurrences” is a much better argument than either of those, though my non-expert impression is that experts don’t generally find it very convincing. “It was probably a natural zoonosis, because if you look at the pattern of early spread it looks much more like it’s centred on the wet market than like it’s centred on the WIV” is also a much better argument than either of those; I’m not sure what the experts make of it. In the absence of more cooperation from the Chinese authorities (and perhaps even with it) I would not expect any argument to be very convincing, because finding this sort of thing out is really difficult.