Build advice, Feral Tiefling Ranger

[Alejandro]

Making a set of PCs with my group for a future game. (We might distribute the PCs among ourselves, so I might not play this specific PC.) One of them is a feral tiefling ranger that will dual wield.

No, not the wings having feral tiefling.
Yes, I am already aware that two weapon fighting is 'bad' in 5e.

Just looking for advice on such a build. Is Horde Breaker or Colossus Slayer a better choice for a dual wielder? Is multiclassing into Rogue, Swashbuckler worth it for its dual wielding features, or no? Etc. Thank you :)

[Specter]

The choice of Hunter's Prey is very campaign dependant. If you know you'll be facing many hordes and groups of enemies a la Lord of the Rings, then go Horde Breaker. If your DM likes boss fights anime style, however, go Colossus Slayer. Neither are bad.

Take Dual Wielder after maxing DEX, and keep in mind that you can't perform somatic components of spells with two weapons in hand, so stow one if needed. Also realize there's no need to take Two-Weapon Fighting as Fighting Style. If you keep your bonus action busy frequently, consider Defense instead.

On multiclassing, Ranger 12/Fighter 4/Rogue 4 is solid, especially because Ranger damage doesn't scale much after level 5. Action Surge, Expertise, Sneak Attack and much more. Take 5 levels in ranger before anything else, though.

As for essential spells, take Hunter's Mark and Pass Without Trace.

[Easy_Lee]

You can actually use a quarterstaff as an arcane focus, though I'm unsure if it can also be used to cast ranger spells. But if so, you could make a strength based ranger with a quarterstaff and longsword, for example.

[gfishfunk]

Generally, its useful to have a good feel for your dm. Does s/he prefer throwing out strong, powerful enemies or hordes? I know its an obvious way to look at it, but under my last DM I would have almost never used Horde Breaker, but Colossus Slayer would come up nearly every other combat.

Colossus Slayer is better with two weapon fighting. The benefit is applied once per turn, so you have a good chance just to apply the damage, so long as the thing is already damaged. This is pretty good in a one monster / two monster fight. Also, there is very little DM dependency here: if the thing is hurt, you add more damage.

At level 4:
Attack
Attack (off hand)
If either hits, +1d8 damage, which is a higher chance to hit

Horde Breaker is better with the Archery combat style or with Dueling. The benefit is applied as an extra attack. That means doing the extra damage is dependent on the attack roll, but you can generally do more damage on a hit. This is pretty good when you have three or more monsters in a fight. But, there is a HUGE amount of DM dependency here: if your GM metagames a little too much (or other less harsh reasons), you cannot use this even WITH a horde because the enemies are not within 5' of each other. They are spread out. I know my last GM purposefully spread out his smaller monsters so that a well placed AoE spell could not kill more than 2. Love it or Hate it, he made a conscience decision that he thought would enhance gameplay that made this kind of thing worthless in practice.

At level 4: (with two weapon fighting)
Attack
Attack (off hand)
Horde Slayer Attack - needs an attack roll + correct positioning to do damage

I like Rogue for multiclassing. I admittedly do not know Swashbuckler. If you really just want damage, go 5 ranger (extra attack) and then as much rogue as you want. Damage is not everything, though, and Ranger is pretty good as is.

[Alejandro]

Generally, its useful to have a good feel for your dm. Does s/he prefer throwing out strong, powerful enemies or hordes? I know its an obvious way to look at it, but under my last DM I would have almost never used Horde Breaker, but Colossus Slayer would come up nearly every other combat.

Colossus Slayer is better with two weapon fighting. The benefit is applied once per turn, so you have a good chance just to apply the damage, so long as the thing is already damaged. This is pretty good in a one monster / two monster fight. Also, there is very little DM dependency here: if the thing is hurt, you add more damage.

At level 4:
Attack
Attack (off hand)
If either hits, +1d8 damage, which is a higher chance to hit

Horde Breaker is better with the Archery combat style or with Dueling. The benefit is applied as an extra attack. That means doing the extra damage is dependent on the attack roll, but you can generally do more damage on a hit. This is pretty good when you have three or more monsters in a fight. But, there is a HUGE amount of DM dependency here: if your GM metagames a little too much (or other less harsh reasons), you cannot use this even WITH a horde because the enemies are not within 5' of each other. They are spread out. I know my last GM purposefully spread out his smaller monsters so that a well placed AoE spell could not kill more than 2. Love it or Hate it, he made a conscience decision that he thought would enhance gameplay that made this kind of thing worthless in practice.

At level 4: (with two weapon fighting)
Attack
Attack (off hand)
Horde Slayer Attack - needs an attack roll + correct positioning to do damage

I like Rogue for multiclassing. I admittedly do not know Swashbuckler. If you really just want damage, go 5 ranger (extra attack) and then as much rogue as you want. Damage is not everything, though, and Ranger is pretty good as is.

The features from Swashbuckler that would come into play are adding Charisma bonus to Initiative (as well as Dex) being able to apply Sneak Attack damage to a target even if one does not have advantage attacking it, as long as none of your allies are adjacent to it, and being able to move away from an enemy without provoking from it, as long as you have attempted (not necessarily hit it with) an attack on it.