Best use for Horizon walker?

[Spacehamster]

Any build that uses the bonus action for a attack is out, so is long range combat(not that big a deal in dungeon crawls where space can be limited anyways).
So I guess that leaves with dueling fighting style with a shield? And maybe crossbow master that uses heavy crossbow but keeps within 30 feet of the enemy and
have 3 levels of scout to help keep the distance from getting in melee perhaps? :)

[Grod_The_Giant]

Either Dueling (in a no-feat game; otherwise Shield Master is probably more useful), a two-handed weapon user (who's not going for Polearm Master), or an archer (who doesn't mind getting close and doesn't go with Crossbow Master). It's not great; luckily, the spell list is, as are the later abilities.

[jaappleton]

The best weapon build?

Well, it depends on how your DM rules.

It’s a bit cheesy, but think about this:

If you teleport behind your enemy, shouldn’t you have Advantage on your attack?

If your DM rules that, GWM becomes an enticing feat. Take the Defense fighting style, grab a Heavy weapon and go to town. I wouldn’t take the GWM Feat until Lv8, though; Etherealness to get around enemies and now you have Misty Step from lv5.