Rackspace is probably the industry leader, but there are a number of smaller companies that will also do a great job for you. Your provider of choice would really depend on your management needs and number of servers. IBM probably won't even talk to you unless you're going to bill more than a million dollars annually.
15 Minute Servers / Net Access Corp.
Rackspace certainly has a name for managed hosting. Their motto is "fanatical support", and I can vouch that they are serious about providing the best support in the industry. However, like the previous poster indicated, there are a number of factors in determining your hosting provider, first of which is your technical requirements and budget. If you want a low-end shared account, then you're going to have to sacrifice on your support requirements.
Here's a direct link to their managed servers FAQ. Specifically, read the thread about "What is Fully Managed?" and you'll get an idea of what they offer.
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