Best SIP Stack for creating .NET application

Hi All,
I would like to develop a Softphone in .NET (C#) and since I am new to SIP client development, I am not sure about choosing a SIP stack.
Can you please let me know the best SIP Stack for .NET?
I have come across a few PJSIP's SIPEK SDK , Independent Soft's SIP.NET, VOIP SIP SDK or PC Best SDK....

PS: the features i need to have are
1) PC to Pc call
2) Pc to Phone call
3) Video call
4) Instant Messaging

I am using one called Ozeki VoIP SIP SDK (http://www.voip-sip-sdk.com/).