The brand of gas geyser I think is best depends on stock availability, warranty and after sales support. So the best brand gas geyser can change at any time. They are: Rinnai, Bosch and Paloma.
I really like Rinnai gas products not only their gas geysers, they are also very well recognized internationally and have some of the best energy ratings, so I recommend Rinnai and their products. I would choose Rinnai if I had a slightly higher budget as I think they have a slightly higher build quality (purely subjective).
Bosch is known for their various other high-quality products and their gas geysers are no exception. I have the most experience with Bosch’s water heaters and I really like them, from there natural draft units up to the Bosch 27L condensing (very nice). Bosch gas water heaters are also very efficient and have top efficiency ratings like the rest of the brands gas geysers on this list. The Bosch Optiflow range come with controllers so they have a great value proposition, they also have a smaller 16L unit that is cheaper if you only require 16L per minute.
Paloma is what I happen to use at my own home and I can say its been a dream. I also have a Paloma space heater which we are very happy with (one of our smarter purchases). They are also priced competitively which is a plus.
Disclaimer: I am not saying that brands not listed here are not good, but if I were to chose these would be the brands I think are the best.