Claude Debussy and Marice Ravel
Ah the original odd couple. Maybe this comparison is the reason why Ravel told everyone he was actually Basque and not French. Or maybe it's why Debussy slept with so many women. Or probably none of that. Debussy and Ravel were both operating at the same time, were both extremely successful in their own lifetimes and both composed in what could be called an impressionistic fashion (although they both apparently hated being called that, more in common!) although they styles diverge after that. Debussy can be thought of as a bit more amorphous while Ravel often composed in stricter tempos with a concrete pulse. Of course, they both dabbled in the opposite but overall Debussy feels more like clouds of notes without an emphasis on melody, whereas Ravel often weaves fine melodies above spinning accompaniment. Entire papers and books have been written on the topic if this strikes your fancy. Does it strike mine? Not really.
Scriabin and Rachmaninoff
I've always found this pairing odd. Scriabin, the doomsday-bringing mystic and Rachmaninoff, whose biggest eccentricity was that he refused to join the 20th century are always paired together in books. They were both Russian, born and active over basically the same period of time, but apart from a contest in which Rachmaninoff placed first and Scriabin second (an actual contest, not the concert-hall popularity contest that was also won by Rachmaninoff) these two had quite divergent lives and musical styles. Scriabin got lost in theosophy (woo cults!) and Rachmaninoff basically had a straightforward life. Scriabin's music sounds like Chopin from the future and Rachmaninoff sounds like Chopin from....Russia. Rachmaninoff was so rooted to the past that he was quoted on saying that 20th century music held nothing for him. Although that didn't stop him from putting on a bunch of concerts of Scriabin's music after his death in 1915.
Webern and Berg
I like to imagine these two students of the Second Viennese School (note: not an actual school) hanging out awkwardly with Arnold Schoenberg in his small flat in Vienna. In my mind the three of them are sitting in the parlor, sipping coffee out of tiny cups and listening to Schoenberg talk about the difference between atonality ("not my bag") and pan tonality ("my bag!"). Webern desperately wants to leave, but Berg is all "but he's our great teacher, we must respect him!"
Ginastera and Albeniz and Granados and De Falla
Schumann and Schubert