Which top engine is strong on what? We select an engine to use, and stick with it. But I heard that all engines have different characteristics, and can be better or worse comparing to each other, in different positions. For example: The champion engine can be beaten by another engine in some specific position. 

So my question is, which engine to use in which positions? 

Thanks in advance.
According to Hartmann's findings in an article in ChessBase: 


  Houdini is a tactical juggernaut. It tends to do best on the various
  tactical test sets that some engine experts have put together, and it
  is fairly quick to see those tactics, making it useful for a quick
  analysis of most positions. Its numerical evaluations also differ from
  other engines in that they are calibrated to specific predicted
  outcomes:
  
  A +1.00 pawn advantage gives a 80% chance of winning the game against
  an equal opponent at blitz time control. At +2.00 the engine will win
  95% of the time, and at +3.00 about 99% of the time. If the advantage
  is +0.50, expect to win nearly 50% of the time. (from the Houdini
  website)


Larry Kaufman, who works on Komodo, said in an interview on the Quality Chess blog that:


  Komodo is best at evaluating middlegame positions accurately once the
  tactics are resolved. Stockfish seems to be best in the endgame and in
  seeing very deep tactics. Houdini is the best at blitz and at seeing
  tactics quickly. Rybka is just obsolete; I like to think of Komodo as
  its spiritual desceendant, since I worked on the evaluation for both,
  although the rest of the engines are not similar. Komodo’s assessment
  of positions is its strong point relative to the other top two, which
  overvalue the queen. Komodo has the best sense for relative piece
  values I think. Komodo is also best at playing the opening when out of
  book very early.


In the article, they conduct some tests using very complex positions, to see if engines can find the solution.