Colossus slayer IMO is simply the best option for any ranger because of its versatility of use. This might be different on release, but currently in EA there are only a few fights where enemies will be gathered in a way horde breaker will be useful. Meanwhile colossus slayer works on anything everywhere not at full health. Similarly Giant killer is not great simply because there aren't many large enemies.
As for hunter's mark I'm not sure you can reapply it to the second horde breaker target, but it costs a BA so if you certainly wouldn't be able to reapply in the same turn you apply it (unless below 50% and wearing helmet of grit).
