Which is the most CPU efficient chess engine? I am learning artificial intelligence and chess programming. I want to analyze the architecture and read the code of existing chess engines. But I am confused between multiple choices. There are several free chess engines available, for example: 


Crafty 
GNU Chess
Stockfish
IPPOLIT
Fruit 2.1
(Add your chess engine here)


I am wondering which chess engine is most efficient in terms of CPU cycles used.
For example, if Crafty achieves 3300 ELO by thinking for 30 seconds on each move, and GNU chess achieves 2500 ELO by thinking 1 second on each move, I would consider GNU Chess to be more efficient.

So which engine is most efficient? What would lead me to choose one over another?
If I were going to do this, I'd choose Stockfish, Fruit 2.1, and Crafty for no reason other than their high quality is proven.  Then I'd compare and contrast the techniques, algorithms, and choices made by each.

These aren't easy programs; it would be a lot of work to gain a true understanding.