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Do you mean this in a stylistic sense, as in the tenth is his only symphony where he writes in a more 20th century style, rather than his usual late 19th century romanticism? Because I can see where you're coming from if so, but if not, chronologically, symphony no. 4 and all those after it were written after 1900, and a good few of those are among the greatest symphonies of all time, let alone the 1900s (nos. 5 and 9 especially) I would argue that should be how we define this question, since after all it's literally "the greatest 20th century symphonies" and those are symphonies written in the 20th century.