1. Server Availability
A website alternates between "uptime" and "downtime" moments. The former must largely dominate so that your site is available as often as possible. If your site is too often out of service, users will get bored - besides your SEO on search engines will be less good. Finally, avoid any web host that does not promise, at least, a "uptime" guarantee of 99.9%.
2. Loading time
Naturally, Internet users hate internet sites taking time before charging. There are two main reasons for this: either the code/site is too heavy or the server is unable to provide the pages properly. In both cases make an evolution of your hosting offer will improve the loading time.
Indeed, just like the availability of the server, the loading time weighs as much on your reputation as on your referencing. The idea is to have its own server maintained by a web agency with hosting skills, which can slightly increase the price of the website - but is actually profitable thereafter.
3. The location of the server
When choosing your web host, go to a server close to you. Most of the time, you can choose where you want to be hosted. This choice is very important, as it will affect the availability and speed of response of the server. Indeed, the more accommodation will be physically close to your visitors the better the performance.
Moreover, if your activity is important and concerns a large geographical area, you can orient yourself towards solutions allowing to choose the best server according to the location of the Internet user.
4. Processor power
The machines used to host your site must be powerful to allow for an optimal loading time. Indeed, the contents PHP and MySQL require a good web host in order to be quickly displayed. The sites are more and more complex, which implies treatment of the server more resource-intensive. If your site is particularly rich and/or traffic-intensive, consider this major factor.
5. The reputation of the host
Some hosters who are unscrupulous in their hosting practices (using malware or publishing illegal content, for example) are automatically blocked on the user's computer or telephone. So, before subscribing to a web host, check his reputation and make sure of his reliability.