Who has the first contact with the client?

3. Always, the Product Owner

Allow me to ask a different question --

Why does it matter?

Important part of the question is First contact.
It is tricky question - client can be not familiar with SCRUM and can try to reach development team. Scrum Master is responsible to teach about interactions with Scrum Team and should shield the team from such distractions.

I guess a am a few few too late to answer this, but anyhows ...

Well to answer this question simply, i believe that it really depends on whether the client is familiar with Agile to start with or not. It can be quite tricky to sell the Idea of agile to any potential client not familiar with the concept of agile.

My Suggestion will be to put your best step forward. Identify the individual in your team that has acute observation skill and a felicity of highlighting agile benefits. Its hard since this requires not only skills and experience in client management, Agile implementation/Agile events but also interpersonal skills. 

So To sum up, any one who is 

• Good with Understanding situational content and draw out benefits of agile in an instant (skill: Observation and persuasion)
• Understand Agile (Frameworks and Methods) and answer client questions precisely (Experience with agile Teams)
• Has Industry Acumen (How the industry works and the tools used in general) 

The person you are looking for is an Agile Coach regardless of what role or job title he has. Find someone who has either the above skills or the potential to adapt and pick up such skills.