It’s konacha. Mainly because its low price, coupled with the fact that in most traditional Japanese restaurants tea is actually free.
Why is it called agari?
To understand the origin of this word we must go back to the Edo period. In Japan’s red light district, guests were always given green tea. The first tea served was called odebana (お出花), which translates as “served flower”. The last tea served was called agaribana (上がり花), meaning the “last flower”. The “flower” refers to the cup.
Apparently the term agari was adopted by sushi bars and it’s still used today.
The function of green tea when eating sushi
You may also notice that green tea in sushi bars is unusually hot. The reasoning behind it is that hot green tea is better at washing away the oils in one’s mouth after eating some types of sushi.
Sushi has a delicate flavor, and it can’t be fully tasted if one’s tongue is still oily from the last sushi eaten. In this sense green tea acts like a cleaning agent, much like gari (marinated ginger) which is also served in sushi bars for this purpose.
If you’re making a sushi party, why not brew some agari? You can buy very economical konacha tea bags, such as these.