I've not tried 'em all, but among the RSS aggregators/newsreaders I've tried on both Windows and Mac platform, Ranchero Software's NetNewsWire is my favourite. Like most Mac apps, it's intuitive and easy to use; has a neat notepad; a decent collection of AppleScripts exist; and had its own Weblog editor.
The folks at Ranchero just announced that they have a public beta of NetNewsWire 2.0 available. I downloaded and gave it a quick look-over. Nice. Biggest and best improvement is the speed at which it and the content you access loads. It's a lot faster.
The other big thing: The Weblog editor within NetNewsWire is gone. Now, when you want to blog something you're reading in NetNewswire, NetNewsWire will hand off some content, with pre-formatted HTML, to your favourite blog editor. (Mine happens to be Ecto on the Mac and on the PC I'm using BlogJet until Ecto for Windows gets some legs.)
The folks at Ranchero are also getting into the blog editor game, with the beta release of MarsEdit. Looks more than a little bit like Ecto's user interface. Haven't tried it yet but will fire that up over the next few days. Love to hear your thoughts on either one of those apps or your other favourite newsreading app or blog editor.
And the other big thing: In NetNewsWire 1 or most other aggregators I've used, you get a headline and a few grafs from the RSS feed. If you want to read me more you click on a link within the aggregator, and presto, you're whisked off to your browser where the Web site loads. In NNW 2.0, though, you get the Web page popping open with NNW. I'm undecided if I like that or not. I think I'll have to get used to it. If you do open the Web page within NNW, it's a simple click or keyboard combo to jump to your default browser and have the Window open. I like that flexibility.