I use Gargrin Zero!! It's good value for money, so I'm only using this.

Gargrin Zero is non-irritating and free of artificial colors. The 1200ml cap actually functions as a cup, so it fits perfectly when poured in. They often have 1+1 promotions, and I think you can even buy it for around 10,000 won during big sales on Coupang. You can get great value for money with the 1+1 deal, and it instantly refreshes the lingering dryness on your tongue. When I can't rush to brush my teeth at work, I use Gargrin and then brush when I get home.