For really good quality I use Skype. It's available for laptop or cell phone and it works great! Minimal delay on a good connection.

I've used most out there. But I think NexGen is the best out there. It can do anything. My second is Wide Orbit. Never a problem and over the last 3 years I've been working with it, never an instance of any hiccups with it. It just runs the way its supposed to 24/7. Simian is also good in my opinion...