The first one is the best suited as there is emphasis on the fact that you were talking exactly about the subject

Of the six, five are valid, the choice between them depends on what you want to say: We were talking about precisely that (i.e. we discussed that exact thing). We were talking about just that (if the stress is on 'that': the same meaning as the above; if the stress is on 'just': that was our only topic of conversation). We were just talking about that (if the stress is on 'just' and 'that': that was our only topic of conversation; if the stress is on 'talking': we talked about that very recently). We were talking precisely about that (if the stress is on 'precisely': when we talked about that, we spoke in a precise way; if the stress is on 'that': we discussed that exact thing). We were talking just about that (that was our only topic of conversation). Notice how in some cases the meaning depends on which word is stressed. There is no way to show the stress in writing, these forms will therefore be ambiguous when written and are best used when speaking.

the first choice is best