Rybka is strongest on the top of the list, but why does it matter? I think even the 20th strongest program can easily destroy 99.99% of the chess players easily. There is no point in playing against them because they play so well that you never had a chance. Rybka went undefeated against many GM's , so there is no point to playing against it unless you are Kasparov or something. I like Deep Sjeng, it is strong, has a great interface, but most importantly I can adjust it's playing strenght to match my own and get better, not get shot. To be honest, that's what really matters.
What do you think is the strongest chess program?