First, I am hoping that you have some fundamental investment strategies completed in place.
Next are foreign currencies. Foreign currencies are not invested for long terms strategies, except in rare cases, or for using such currencies to their home markets where you have a lot of business and will be using the cash.
The best strategy is to buy foreign currencies that are on the rise for at least 52 days of strength with the appropriate margin that you are looking for. Use a foreign trading discount brokerage that has the tools for this. Remember, that you are doing this on volume, so small amounts of currency will not serve you well.
But I would be doing you a disservice if I mentioned a specific currency. Why? I do not currently have any holdings in any foreign currencies at this point.
Best of luck.