Best SIP SDK With Asterisk


I have multiple installations of Asterisk running at different locations, clients are using different SIP Phones (Hardware/Software) but I have noticed that the voice quality on Hardware based IP Phones /ATAs is always better than Softphones

Can somebody tell me what are the reasons behind it and which SIP Phone or SDK is best for SIP Client with Asterisk,currently I am using Microsoft RTC Client API for customized SIP Clients