What is the best resolution for my video?

Always try to upload your video at the highest resolution possible, as YouTube will automatically serve a lower or higher resolution depending on the user bandwidth, screen size and other resources.

According to the YouTube documentation at https://developers.google.com/youtube/iframe_api_reference#Playback_quality

YouTube [...] select the most appropriate playback quality, which will vary for different users, videos, systems and other playback conditions.