Here’s a site that lists the uptime of VPS providers. From my experience, Knownhost is solid (although pricier) for a VPS and they currently have a 99.983% uptime listed. They are a managed VPS provider, but they also added an unmanaged option via RocketVPS. Uptime is one of the major factors when picking a VPS — along with RAM, bandwidth, and CPU priority.
http://www.hyperspin.com/en/ranking.php?type=2&i=10&d=90&s=10
