Looking into streaming movies from the likes of Amazon,iTunes, Wuaki etc.

Does anyone know the quality of each of these and who is best quality wise?

Are you looking at pay-as-you-go (as in renting) (since you mention iTunes) or subscription VOD such as Amazon Prime, Netflix or Wuaki?

I'm using Netflix and iTunes via Apple TV and they're both pretty excellent.
For the purchases of most movies via AppleTV they come with iTunes Extras which is just like having the Blu-ray copy as you've got the same sort of title menu and special features.
Obviously Netflix is a subscription service whereas movies via iTunes is a buy the keep option & also renting per movie option.

Interested in the rental services for films rather than Netflix etc.