The best replacement for stockfish is rybka.

The best alternatives to stockfish are rated as following:
★★★★★  rybka
★★★★★  houdini
★★★★  hiarcs
★★★  critter
★★★  komodo
★★  smallfish
★★  shredder
★★  crafty

Your alternative for stockfish is not listed? Suggest!
Speaking about stockfish, one probably shouldn't forget about xboard.

Other search terms related to stockfish and their ratings are:
★★★★★ xboard
You have in mind a companion for stockfish and it is not yet here? Propose!



Google Trends

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stockfish
rybka
houdini