So I looked into all of these some time ago for an app that I use myself. The Safari fork of Readability is the best client implementation, while the best server-side implementation (by a mile) is Diffbot.

Out of the 50 sources I had (regular RSS updates, etc.) InstaPaper failed on far too many, either stripping important content or pictures and captions. Readability was only a bit better

I glued together an app that will pull in RSS, send each URL to diffbot, and then collate the output into a reader that I use. Never thought of releasing it as an app, I just use it myself.