The worst thing he can/could call you is a coward.

[MichaelLu2000]

1. The worst thing he can call you is a coward.
2. The worst thing he could call you is a coward.

Which one is correct or better?

 

[emsr2d2]

They're both grammatically correct and possible. It depends on context, as always.

 

[MichaelLu2000]

They're both grammatically correct and possible. It depends on context, as always.

What’s the difference?

 

[emsr2d2]

What’s the difference?

Context! As I already said. Why don't you give us a situation in which you might want to use the sentence and then we can see whether we would choose "can" or "could"?