When it comes to deciding on the best gas water heater it sometimes helps to reformulate the question a little. For example we could just add ...for my needs at the end.
Tankless hot water heaters are like many things in life - there is no clear answer to the best question. The more you pay, the better they get, very roughly speaking. But better for what? If you have 20 square yards of lawn would you really bother buying a sit-on mower?
When it comes to on demand water heaters there are some sensible minimums to bear in mind. These units are rated by how much water flow they can heat - this can be translated into household water demanding units (washing machines, faucets and showers, mainly).
So before even looking at tankless hot water heaters decide what you need them to do. How many appliances do you use at any one time? It does not matter if you use one after the other, since these units can keep heating water at their maximum rate for ever. So if you habitually use two showers whilst the dishwasher is running then you are looking for a unit that can match this demand.
There are many instant hot water heaters that can do this, and often the capacity is expressed in terms of how many appliances it can handle. Like most things you probably want a unit that can meet and slightly exceed you maximum. But we should all be trying to use less water in any case, so there is no point getting a unit that could serve a hospital if you are four person family!