Originally Posted by Asher ross What the key factors which are really required to measure quality and reliability of a web hosting service?

Your signature points to a web hosting provider and you should be answering such questions. I'm sorry but such a question is quite naive when it comes form a hosting provider rep. It looks like created from a "hired forum poster".

On the topic: i would suggest one to use both external and internal (inside the network where the sites/servers are hosted) monitoring tools.