You've probably already answered this, but what is the best response for the tired argument that atheists can't possibly have any morals?

There are probably plenty of "snappy" answers to a statement like this, but I'll give you a more practical answer instead.

If someone said to me that atheists can't have any morals, I'd respond that I think atheists can be as moral as anyone else and then ask why the speaker disagrees. The response to the question will dictate how the conversation continues. For example:

1) If the speaker answers that all morality comes from God, then I would argue that this is not the case (which it pretty clearly isn't, but that's a whole other subject).

2) If the speaker answers that one cannot be moral without fear of divine punishment, I would ask if the speaker would be immoral if there was no fear of being caught. If the questioner says yes, I'd say that I think that's a pretty sad attitude.

3) If the speaker answers that morality can't be learned without religion, I would explain how I arrived at my morality without any assistance from religion.