A host that offers a 100% uptime guarantee, or one with a record of having that level of uptime?
An uptime guarantee hardly means much, unless there is actually some incentive in place they can offer you when their servers go down. Also keep in mind that hosts don't usually include server maintainance in their claimed uptime.
And from my experience with Site5, their uptime has been great.
What I mean is one that offers 100% uptime excluding maint that offers an incentive. I agree with you,
"An uptime guarantee hardly means much, unless there is actually some incentive in place"
If we did not offer an incentive I could easily go on our website and say 100%, but since we do offer a prorated bill we give the room of an unexpected outage of 1% downtime. Anything beyond that is compensated.