the criteria to be chosen as the the most important chess engine could be something like

influential that is
it exerted a lot of influence over other engines

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powerful with high dominating elo

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a new mechanism used
like rick48 like it is said to be not using alpha-beta algorithm
but active space
(the info about rick48 and active space are scanty and rare
so I don't know exactly what these mean but)
it looks like the engine doesn't search like all other engines
like searching deep with brute force of CPU power
but rick48 can positionally search and think like humans

http://chessprogramming.wikispaces.com/Stanislaw+W.+Rostkowski

version w4 announced to become commercial [4] [5], which is under development for almost 25 years from version a0, ..., z0, a1, ..., w4. As mentioned on its site, Rick48 employs a new algorithm dubbed Active Space Algorithm (ASA) [6].

http://rick48.com/shop

We still work on multicore version and then we'll send it to IPON, CCRL, CEGT for tests. We expect it should be done within 2013. Rick's Team

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the engine can play like humans
or close to playing like humans
with positional awareness and tactical planning