For the most part tank water heaters are all the same so it comes down to a preference, I normally use AO Smith or Rheem. Not because they are necessarily the best of the best, but because of price and availability along with the quality. If I can get good quality, quicker and for a better price, then I can serve my customers better.
Tankless Water Heaters
For Tankless I choose Rinnai. The installation is straight forward and I have yet to have problems with the one I have installed in my own home.