What's the best format to use for YouTube?

YouTube has introduced HD.

Use Kdenlive 0.7.1 as H.264 export is better. Refer to this document:
http://www.google.com/support/youtube/bin/answer.py?hl=en&answer=132460

These export profiles has been added to Kdenlive recently:

YouTube 1290x720 2000k
YouTube 640x480 800k

Please note:

It is recommended to create a project with the same resolution as the target export resolution.