Do you have a preferred altcoin you think we should take in the account in to use as a payment method and why?
Many altcoins can be used as payment method, even bitcoin can. Above I might gave a bit "bad overview" of Nano, but honestly speaking Nano can be used as good medium of exchange in terms of freeless and instant transactions.
Another one that has been long time till now would be like stellar lumens as it also has pretty fast transaction with low fees.
Maybe steem since transactions are free and block time is 3seconds. But disadvantage here is that you will have to have an account (for which you have to pay for or you apply for steem team to create you one which can take long time). Then with an account it is much easier to track where is money moving so not the best in terms of anonymity.
Other than "ordinary" alts, maybe stable coins will play bigger role as they are designed to have stable price. Since price is stable hopefully they could potentially be used more often to transfer value.