This question can be answered simply:
Full HD or UHD - exported as an MP4 or MOV
Note: Best quality for playback will be through Vimeo - they have fantastic capabilities and you can embed your uploaded video directly to our site - this is the recommended and best option. They have their own list of requirements if you are looking for their details.
or if you prefer the complex answer and want great quality:
1080P (1920x1080) // Codec: H.264 // Frame rate as shot // VBR 2 Pass // Target Bitrate: 12Mbps // Maximum Bitrate: 25MBPS // Maximum Render Quality // Level 4.0 or higher // Stereo Audio
UHD (3840x2160) // Codec: H.264 // Frame rate as shot // VBR 2 Pass // Target Bitrate: 25Mbps // Maximum Bitrate: 50MBPS // Maximum Render Quality Checked // Level 5.0 or higher // Stereo Audio
Or if you are exporting in HEVC everything changes quite a bit - but I'm sure there are dozens of resources that will allow you to choose the proper compression for a delivery codec.