This seems to be a good guide to a variety of instant hot water heaters. For some reason they have not listed Paloma water heaters — we have had one for 25 years and it still works fine. However, it has a pilot light, and many of the new ones don’t. If this one wears out, I’ll get one sans pilot light.
Tankless water heater guide: https://www.tanklesswaterheaterguide.com/
Paloma tankless water heaters: https://www.palomatankless.com/