Google Reader is the RSS tool of choice for most of the residents of our office – and with good reason. It's fast, easy to use and extremely reliable.
New posts typically appear with commendable speed, and the only real fly in the ointment is that Google's search feeds (which send you any new mentions of the websites, people, companies and products you specify) has recently been replaced with a truly horrific system that can't distinguish between a link in the sidebar and a mention in a post. That makes it hideous for bloggers and researchers alike.
Still, it's a small price to pay for the rest of the system. Throw it into List mode and you can easily scan titles for something of interest, while the river of news in Expanded mode gives you the full scoop.
As you'd expect from a Google product, searching through feeds couldn't be easier, whether you're simply looking at your personal subscriptions or browsing a wider field. The mobile client is also excellent, particularly the custom version for the iPhone. If you choose to use only one RSS reader, you should make it this one.