I've used quite a few different ones over the years. I can tell you that I recently got the Plantronics K100 speakerphone and that one sucked. It kept cutting out on the callers voice when I was using it.
The Jawbone Icon is pretty good.
Right now I'm using the Motorola Oasis and, I have to say, it's working quite well. It's not the prettiest (in my opinion) but it does what it's supposed to do. I've had zero complaints using it so far.