Have tried and tested:

Google Reader - part of the Google package and comes as an extra with your google mail option. Nice reader that is easily usable on all sites offering RSS - love the one stop shop with same login and password for mail, docs, reader etc
My real PICK of the bunch is most defiantly Socialite. It's an app that sits on your computer, a client if you like - BUT with this little beauty it has my RSS reader included, my Flickr accounts, my Facebook updates AND my Twitter - in one awesome package - no need to flick from page to page or site to site. Love this little beauty and well worth the minimal outlay! Having trialled many other Twitter clients this baby is the one that has me hooked with everything in the same place. Could not live without it!
My advice - try out a few of them and see which one works for you!
Have fun trialling! Let me know which ones you find and what you think of each one.