Do Any One Know Which Brands of IP Phones Have Better Voice Quality and More Functions Comparing to Others?

The higher quality IP-Phones typically are priced starting around $80 US. and above (Linksys and Polycom). Grandstream has phones starting in the mid $50 range and up. However, I can't speak from experience as to the quality of Grandstream products.

I use Linksys ATA phone adapters like the PAP2T-NA and SPA2102-NA with good results. Linksys is owned by Cisco which is well known for their quality products. I expect the Linksys IP-Phones use the exact same internal firmware and chipsets as their ATA's.