I've seen great work by all of the above. Here are a few more options:
- Marketo also provides in-house design services. To inquire, you can email to services@marketo.com
- If you'd rather go the agency route, one of my clients had some amazing work done by Red Clay Interactive, including responsive landing pages.
- If you prefer a US based freeleancer, check out www.mikejorritsma.com. He has done some amazing work with responsive design for Auction.com. One of the coolest things I've seen was a grid-based responsive landing page template that can be populated via tokens in 4-column, 3-column, 2-column and 1-column configurations (all in one single template, controlled by tokens).