NP gain

There is A LOT of factors that go into this. the most generic answer is of course a triple arts brave chain. that said, the next most generic is art first card+whatever cards give the most hits, except buster, with some exceptions. all in all its really a servant by servant basis. that said, someone did all the work already, so you can always look up which servant in particular you need to know the chain for:
http://fate-go.cirnopedia.org/command_cards.php just plug in the servant to find their np gain for each combination.