Best format for YouTube?

What's the best video format for uploading to YouTube?

I sent a link to my family for some personal videos, and they said they couldn't watch them because they needed a plugin. I thought all YouTube videos were converted to the same standard format, but now I think they maintain their formatting (mp4, flv, avi, mpg...)

So...

What is everyone else using?

Obviously, if different plugins are needed for different videos that are uploaded, we'd want to be using the one that is the most widely used and hopefully preconfigured for most browsers.

Using flv here..

Flv is the format youtube uses.

When I download videos I want to edit off yt I always make sure to download the mp4 360 version and find it way easier to process for my own taste... Mileage might vary...

I use mp4. It gets you the best quality in the least possible size.

im using mp4, easiest to edit and upload

mp4 or flv