Quote from intradaybill:
Weaken against the ruble you mean?
LOL!
I was going to say what short&naked said - there's no such thing as "I know the (insert instrument here) will go (insert direction)."
But if we assume, dangerously, that you're right, then I - personally - would go long EUR/USD since it would probably be the benefit of any dollar crash. But if you're just going to speculate that the dollar will weaken against "some currency", then it's rather important to know what that currency is, don't you think?
If all you're talking is the dollar index, then pick the Euro, as it is the majority weighted.