hyperspin is the best one I have tested out of 10 or so as far as being reliable and not reporting false alarms, keep in mind if you use the free option you will get 5-30 minute tests at most places that will give you an idea of uptime but will not be 100% accurate unless you upgrade to paid and use 1 minute monitoring, this is what I use and gives the best indication of true uptime, a lot of hosts use the 5-30 minute because they dont wanna pay for 1 minute monitoring or they are scared of 1 minute monitoring as it is the most accurate in reporting their uptime, just depends how accurate you wan't your results, if not a big deal use the free option, if you wan't the most accurate use 1 minute, heard alertra is good as well but never got around to them yet, the only 1 that really showed me false reports was site uptime.