I'm wondering how to get the highest quality single frame extraction from 5D video footage. Any thoughts?

If you use FCP just export as still, and choose as photoshop or Tiff.

Most editing programs will have a similar export feature.

If you're on a Mac with Quicktime Pro, the Quicktime player will do this. Just find the frame you want, type an 'i', then type the right arrow key to move to the next frame and type an 'o'. Then Choose "Export" from the file menu. And choose these two options.

http://images.stevemaller.com/video/QuickTime%20Player-Snapz-001.jpg
http://images.stevemaller.com/video/QuickTime%20Player-Snapz-002.jpg

If you're using Vegas, then set preview window to Best-Full, then click the save icon.

If you have CS4 Photoshop, you can open up native H.264 files, scroll, edit, retouch and save to what ever format needed.

Thanks folks.