First things first: Hunter's Mark is definitely the first spell ANY Ranger should learn. That'll keep you from falling terribly far behind DPR-wise.
As for the Hunter features:
- Giant Killer is probably the best choice overall, and good as is. An extra attack is always good (moreso with Hunter's Mark), and a lot of the high-CR enemies are Large or larger. Basically they eat your attack if they stand next to you and attack you, or they try to move away and you get an OA.
- Colossus Slayer probably needs a boost either to 2d8, or perhaps just let it apply to all attacks you make a la Paladin's Improved Divine Smite.
- Hordebreaker's distance should probably be increased to 10 feet. That'll give it some more utility ... or at least as much utility as a suboptimal attack strategy will allow.
- Said suboptimal attack strategy is also kind of a problem with Multiattack in general. I mean, yeah, I see they were trying to give the Ranger a minor AoE he could do at-will, but the damage for that is just pitiful.
The Ranger is lacking in burst damage, more than anything. Even Swift Quiver doesn't help a ton in that department, it just brings your DPR barely above that of just a Fighter with 4 attacks (plus it's concentration and thus won't stack with Hunter's Mark).
I will say, however, that 5E's Ranger is easily the most evocative flavor-wise. A shame its combat effectiveness doesn't quite match.