What features do you need and how much are you willing to spend? Many high-end phones include:
Direct IP calling. No issues with DTMF, talkoff, doubletalk, echo heard by remote party, etc.
Concurrent calls. When one handset is in use, you can make or receive a call on any other.
HD voice. Usually limited to intercom and calls between users of the same provider, though sometimes also available between different providers via SIP URI or iNum.
Support for connection of a wired and/or Bluetooth headset.
Cellular integration, as described by RonR.
Speakerphone. They aren't very good, but still useful for hearing an IVR while entering digits, or when waiting on hold for a long time.
Non-voice functions, e.g. send and receive SMS, view Internet content, etc.
Contact lists, call history, etc., comparable to modern smartphones.
You might look at Panasonic KX-TGP551T04, Gigaset C610A IP, Gigaset A580 IP, other handset models compatible with these bases.