catastrophegirl's roku channel list: free lists 861 free Roku channels as of today, 5/18/13. Also, "Best" is kind of subjective.
I don't know what kind of free content you're looking for, movies or TV, but here's some of my favorites:
Plex (You need to run Plex server on a computer for this)
Live News (live streams from Al Jazeera English and DW networks, among others)
I also use PlayOn, and there's a lot of free content there, it's free once you've paid for the PlayOn app for your computer. Roku owners can get a lifetime license for Playon for $39.95 by following this link: PlayOn lifetime license $39.95 for Roku owners