Without a doubt:
http://www.panic.com/unison/

That, however, is mac only. Are you using a macintosh computer, or a windoze?

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I'm in the Windows camp, with no plans to switch alliances to the Jobs world of Apple...but I have considered switiching to or at least dual booting a linux distribution...fedora or unbuntu...

Apple isn't a Jobs world d-: We don't all worship him as a god. I personally don't actually like him at all. I love the Apple products though.

In any case, I don't know any windows usenet readers...I could google a few, but I couldn't in good faith recommend them without using them )-: Sorry.

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I use easynews. They have a global search that is really nice. And a web interface wich is good for binaries.

Haven't tried others though...

For speed , the NNTP server run by ones respective ISP should generally be the fastest.
Obvious problems the are: loss of anonymity, the fact that they might not carry your favorite groups.

For clients, depends a lot on what you do.
For actually reading and participating in conversations, command-line clients tend to be the best once you get the hang of them. Tin is easier to learn, while nn is more powerful once mastered.

If a GUI is preferred, both ThunderBird and Opera work nicely.

If using newsgroups mostly for binaries: while the above work, when downloading several gigs of stuff a day, both Xnews and XanaNews have served me well.

I use Google groups. It's easy and you can access your usenet groups from any computer anywhere. It's not feature rich like standalone programs, but I like it.

On Windows, I use GrabIt. It is free and works pretty darn well.
http://www.shemes.com/

Alt.binz is a good program aimed at using nzbs. Basically these allow you to search sites that list news articles and download compilations. Its a free program so its worth a try, it cuts down on how much time you spend downloading headers.