If you're looking for Magento hosting, it is definitely key to find a web host that is optimized for Magento. However, many hosts claim that they are optimized for Magento and this is little more than window dressing. To really get the most out of Magento, sysadmins really need to understand how this eCommerce platform works and know every little trick to make your online store go as fast as possible. On the list of hosts that the previous answerer provided, Nexcess fits this bill. They are innovators in the Magento world, having created the Turpentine extension that improves Magento's compatibility with Varnish (which for the non-technical, this is a very good thing). They speak regularly at Magento conferences and have published a whitepaper that is a blue print for improving Magento site speed. I've included a link here. http://www.nexcess.net/magento-best-pra ... whitepaper
Ultimately though, the Nexcess staff is what puts them over the top. They are knowledgeable, friendly, and always quick to respond. I hope this helps. Happy hunting!