I don't know if it's "the best", I'm far from a weather expert, but I typically use accuweather.com because it's what the Forecastfox Firefox extension I use uses. It keeps the current conditions along with the forecast for today and tomorrow at the bottom of my web browser. It pops up a message any time a weather warning is in effect and I can click on it to go right to what the warning is. One click and it takes me to the full forecast with maps, etc.
Other neat stuff you can see in screenshots on the site:
http://forecastfox.mozdev.org/ and of course if you're still using Internet Explorer, you'll first need:
http://getfirefox.com
