Tempura is one of the top favorites among Japanese dishes, also for non-Japanese. What is more delicious than to have prawns, fish and vegetables served in those wonderful golden clouds of deep-fried dough? So it is a good thing tempura goes well with sake, and the choice this time is easy. When you remember that one of the properties of junmai sake is to cleanse the mouth from oily foods, the choice is soon made. Of course, good tempura is not really very oily, but it has been doused in a bath of hot oil. A great variety of ingredients is used, but we do not have to worry about that when making the sake pairing, as the golden dough makes everything equal.
The thing to pay attention to when eating tempura, by the way, is absolute freshness. Tempura should be eaten within minutes (if not seconds) after being fried. If you wait too long, it gets sodden, as is sadly the case when you get tempura donburi-style on rice. That is why specialist tempura restaurants serve the tempura not everything at the same time on a big plate, but bit by bit.
The only thing to pay attention to when making pairings is the way the tempura is eaten. There are two ways: with some salt (often flavored with green tea powder), or by dipping in tentsuyu (3 parts dashi, 1 part mirin, 1 part soy sauce), to which grated radish has been added (daikon-oroshi). The first way is how Japanese connoisseurs eat tempura, the second way is more common and easier to combine with sake.
In the first case (salt) I would suggest a sweetish sake, for example as made on the Inland Sea coast of Okayama or Hiroshima (Hiroshima has soft water which makes the sake sweet). And to add some zest to the dryness of the flavored salt, perhaps a fresh type like a Shiboritate.
But most people will use the tentsuyu dipping sauce, which I think is a good idea (despite what connoisseurs may think!), as the daikon adds sweetness to the umami of the sauce (no, Japanese radishes are not spicy!), creating a perfect combination with the tempura. When pairing, the sturdier junmai sakes are best, those with a somewhat higher level of acidity. Also Kimoto and Yamahai type junmai sakes will be perfect thanks to the slight touch of bitterness and higher acidity. They are a "melting accompaniment to the delicate oiliness of good tempura" as Philip Harper phrases it in The Book of Sake (Tokyo, 2006).
Finally, I would suggest to try the junmai warm, at about 40 degrees Celsius, for even more melting goodness.