There is really no way to state objectively what the greatest YouTube video is, however, it is possible to state what the most popular youtube video is based on the number of views, and that video is called Gangnam Style. Personally, I found it to be only mildly amusing.
You would have to download the YouTube video. Unless the video is under a Creative Commons license, downloading a YouTube video is against the YouTube Terms of Service.
I would upload it to youtube, then use the embed code that youtube puts next to the video description.
You would have to use the YouTube App for iPhones. You will not be able to upload a video from a mobile browser.
It would depend on which video it would be. But I would suggest using Youtube.
There are already things like that on Youtube.
As Youtube's reason to exist is to host and distribute video clips, this would appear to be a very unlikely course of action at any time.
One could watch the video for "The Greatest" by Cat Power on YouTube. More information about Cat Power, including tour dates, can be found online at Cat Power Music.
You would have to use that on your video's editing account.
Once you upload a video onto YouTube, it cannot really be edited any more. You can only add annotations to the video, add branding, and/or replace the audio of your video with pre-approved YouTube audio. If you want to trim a song (which might still get you a copyright strike), it would be best to do it in video editing software before uploading the video onto YouTube.
Some content on YouTube is created only with audio without any video, so you would only be able to hear it. If all the videos on YouTube are white, I would re-install your Flash plug-in or your computer's video drivers.
If there is paint in a video of yours that you would like to remove, you'd have to edit your video using a proper movie editing software. The YouTube editor is unable to remove most imperfections from a video for you.
You would have to use a video editing software to do that. If you don't have that sort of software, the YouTube Video Editor built into the YouTube website now has a blur tool which is very effective at blurring moving objects.