what is a Better Base class for my Horizon walker Build

[Nosta]

Slayer

or

Unchained rogue

I am going for a wandering swordsmen type character and want to use Dimensional feats with to get off sneak attack easily

[Forrestfire]

Unchained rogue, thanks to the Dex-SADness, the buffs to sneak attack, and the existence of Debilitating Injury, is significantly stronger than the Slayer in almost all cases. Urogue 4+ with Step Up/Press to the Wall means you can reliably trigger the massive numbers boost for the rest of your attacks, or keep yourself safe by penalizing rhe enemy's attacks.