If the majority of your audience accesses your video through their phone, vertical video is best. If your videos are mostly viewed on a desktop or TV, horizontal video is the right choice.
If you are planning to publish your video only on social media like Facebook, Instagram, TikTok, or Snapchat vertical video will fit in with those platforms best. If you are publishing to YouTube or other platforms more regularly accessed by desktop computers and televisions, horizontal video will fit those devices better.