I vote for Steinitz. In the Romantic era, he pointed out that solidity and defensive play were not only valid, but superior to the raging fashion of his day.
Who was the most influential chess thinker
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i think tarrasch kind of modified steinitz's rules and then added some of his own. morphy also deserves mention for his emphasis of the importanc of quick development. perhaps som positional masters showed the importance of endgame knowledge. then there are inimitable geniuses like alekhine who could just play well.
I think it was Siegbert Tarrasch.