Depending on the eventual region you intend to model in, there are quite a few brands you could go with that would all be excellent quality.
If you're doing North America or Japan, I'd recommend Kato. There's less available in Japanese prototypes, but plenty of American/Canadian prototypes available in H0 scale.
If you're doing European models primarily, there are a couple of KATO models, but the big manufacturers would be Fleischmann, Roco, Trix, Marklin, and Rivarossi.
If you do go the Europe route, keep in mind that Marklin's models are 3-Rail AC, while most H0 scale equipment is 2-Rail DC. This has both advantages and disadvantages. 3-Rail is not as realistic, but Marklin gets away with it because they have a phantom third rail. 3-Rail is also much simpler to wire for things such as block detection, reversing loops, and circuit/activation tracks compared to 2-Rail. 2-Rail is much more prevalent, and more realistic, but is more difficult to wire due to reversing loops needing to be isolated and having polarity on the mainline switched.
Digital control also differs between 2-Rail and 3-Rail. Marklin has their own "Marklin Digital" system for their 3-Rail trains, while most 2-Rail digital control is handled by DCC, which is an open standard, meaning more flexibility from manufacturers.