When I saw the thread title I was going to post "Google Reader ftw" but I see that's where you're coming from. I can recommend a few good ones for Mac, but sadly that's one of the areas I have no experience at all in on Windows. Google Reader is nice for me because it has a version that tailors itself to the iPhone when I'm reading on there, so my desktop and iPhone stay in sync and I never have to read the same article twice.
However, one of the Mac clients I liked a lot has a Windows "variant," and apparently even has an online version that works with mobile devices like the iPhone. It's called NetNewsWire, but the Windows version is called FeedDemon. Not quite sure how the two compare, but I like NetNewsWire alright, so it can't hurt to try it out.
It's available here:
http://www.newsgator.com/Individuals...n/Default.aspx
Do let me know if it's any good.