C++

If you're talking about the .NET version of C++, known as C++/CLI, then there's very little difference in execution speed between it and C#. C++/CLI may have a slight edge as the MSIL compiler is said to do more optimizations than the C# compiler does but I've never noticed much difference myself.

However, if you're talking about unmanaged C++, this definitely has the edge in execution speed over C# since the compiler produces native code (not MSIL which needs to be converted to native code 'on the fly') and can certainly apply more optimizations to that code than the 'just in time' compiler can.

The speed difference is likely to reduce or disappear altogether in future now that '.NET Native' is with us. Currently, this can only be used to 'speed up' Windows Store apps but it is to be extended to other types of .NET applications as well. Basically, the .NET Native compiler produces native code rather than MSIL and shares optimization technology with VC++.

Of course, execution speed is not the 'be all and end all' of application development. You can certainly develop applications more quickly with C# than you can with C++ as, for one thing, you don't have to worry about memory management with the former. Unless you are writing games, 'real time' applications or 'low level' stuff such as device drivers, ease of development and maintenance combined with reasonable execution speed may be a more important factor in deciding which language to use.