None of your requirements, at least when they're as unqualified as they are, can help you to pick an OS.

Ask yourself where your bottleneck will be, then ask yourself if the OS is going to make any difference there. The answer is very likely no.

No modern OS is going to give you any significant performance/cost advantage over another in serving regular web apps. Most of your trouble will be in setting it up and maintaining it, so go ahead and pick whatever you're most comfortable, or, if you're not comfortable, ask someone who are (and that you trust).

Also, don't pick CentOS, unless you need RH compatibility.