If you have the resources and bandwidth, 1080p. A lot of people watch youtube on large monitors, and the difference between 720p and 1080p is very noticeable. Also, even if people have lower resolution monitors, they set the video to 1080p, and their broswer/ YouTube will oversample the video, and assuming you have sufficient internet speed, it will be crisper. Eg, Setting the video to 1080p on a 720p monitor is still better quality than 720p on a 720p monitor.