Anything over 95 percent is good. But the most important thing I feel is that is the at any instant of time the site should not be down for more than a few minutes.
I'll put it this way. 99% means in 100 days 1 day can be down. Image you are X'mas site and Oct,Nov, december the site is down only for one day between 15th and 25th...Think how much your sales would be affected.... But the host can proudly say that we have given 99% uptime... It doesn't matter to you that time...
At the same time suppose you have a 5 minute reboot every 5 days... It's doesn't matter many of us would not even notice it..
So I'd always suggest a host who has more stable servers with short downtime periods if any... Also the host must always do upgrades shifts during the non peak hours...
Have a great day