Rybka 2.2 is the strongest of those. But you can simply download and install much better engines like Houdini, Critter and Stockfish for your Arena 3.
All of those stomp the Engines coming with Arena 3 into the ground.
Arena 3 engines
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It seems that for houdini only the 1.5 version is free, is it stronger than rybka 2.2?
What should be the strongest engine among these?
SOS 5.1, AnMon 5.75, Hermann 2.6, Ruffian 1.05, Rybka 2.2 free, Spike 1.2