I've found the 'D' to be most useful - it's easy enough to play in G and Em. As a classically trained flautist I can extend the range higher than 'normal' by using flute-style cross-fingerings.

One word of advice - when playing the different octaves, lowest D (on the 'D' whistle) is all six holes covered, the next octave up is five holes - leave the one nearest your mouth uncovered, and the next octave two fingers only - no right hand fingers (assuming you play conventionally with your left hand nearer your mouth).

The notes blow easier, and are in tune.

  < mouth end
  Low D: * * * * * *
  Mid D: o * * * * *
  Hi  D: o * * o o o