Your questions is too broad. You need to narrow it. What platform are you using? Windows or Linux? How about the specification of the server?
To be precise with your question. I would suggest many top companies which are providing cloud servers. But all of these providers will be suitable to you, unless you don't have location and configuration preferences. http://www.rackspace.com/cloud/servers http://www2.iweb.com/cloud-servers/ These are one of the top cloud server providers .
There's no ultimate answer for that. It depends on what you need and how you will use the server. You need to specify ur requirement before picking the right cloud services.
As mentioned by folks above you will need to specify your server specs and/or target location. If you are targeting specific audiences e.g. UK then it is recommended to go for a UK cloud hosting server etc. Most important thing is to look for hosting company that offers 24/7 support.
IMO, no "best" cloud servers, but cloud server that "best" suit to your own requirements. Each of the cloud platform have their own strength and weakness. You may need to compare each of one that meet your own requirements.
Preferably go for UNIX Server which runs low configuration Hardwares also.... Windows has much bugs and slow as compared to Unix nowdays and much cheaper softwares with this kind of servers.
I would say that Cloud infrastructure create with VMware is probably the most reliable one available on the market. VMware based Cloud instances usually feature services like High Availability and Failover.