What is the best brand for OO locomotives?

There are lots of manufacturers making OO gauge engines. The most common ones are Hornby, Bachmann but there are also a plethora of smaller actors such as Heljan or Dapol.

You can also find engines sold exclusively by certain shops under their own name but manufactured by an established brand or even designed in house and made in a factory in China.

To go back yo the initial question, there is no "best brand". It depends on your needs: do you need a less detailed but robust engine for a 5 year old? Do you want the most accurate Class 52 available? Do you prefer the latest tooling or are you happy with something designed a few years ago? Do you require DCC sockets or do you plan to run it only on analogue?

Some manufacturers have sub-brands such as "railroad" which is the entry level range for Hornby. There is nothing wrong for these items but they are not for the collector who wants the most detailed item.

The best brand for you is the one that answers your needs. Do not spend too much time considering the brand but spend time looking at the engine, see it working in front of you and ask information from your model shop. This will help you decide what is best for you.