If you're talking about buying, then it's whatever currency you currently have. If you have multiple, then buy with whatever you think is going to inflate the most in the future.

If you're wanting to sell to buy stuff now, then sell for whatever currency you can use locally.

If you're wanting to seell because you think BTC is about to inflate, then sell to whatever currency you think is most inflation-resistant.


Basically, all FOREX trading (including BTC) boils down to taking your money and leaving it sit in the currency that is deflating the most or inflating the least. Change as necessary whenever you expect the currency you're holding to inflate faster than another currency.