I find the best programmers know C, C++, C#, and Objective-C. They know the pros and pitfalls of each language:

  1) Procedural vs OO
  2) Manual vs automatic memory management
  3) Naked pointers vs smart pointers vs garbage collection vs reference counting
  4) Method calling vs message passing
  5) C libraries vs C++ libraries vs .NET frameworks vs Cocoa frameworks
  6) Zero runtime vs thin runtime vs rich runtime