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Gen I had all the classic tunes and arrangements firmly embedded into our minds. The simplicity of just 3 track music on the Gameboy sounded so good. (Bike Theme, Pallet Town, Cerulean City, Trainer Theme)
Gen II's music for just Johto fit perfectly and sounded like an improvement. GBC's music sounded slightly different with different arrangements and sounds that fit just for Johto, however it failed when trying to recreating the same kind of magic and simplicity of Kanto's themes; like a cheese grater to the sound and quality of the classic songs. (Violet City, Champion Theme, Ecruteak City)
Gen III's music remade all of Kanto's themes for the current standard but had terrible rearrangements and added in sequences that made most of the songs on it terrible, however, most Hoenn songs came out on top with their beautiful melodies and trumpet touch ups. Lets not forget Mirror B theme is also in this category. (Mirror B theme, Sootopolis, RSE Gym Leader)
Gen IV had perfect harmony and atmosphere for Sinnoh. Refreshing arrangements and melodies kept Sinnoh's music diverse, however, Johto's arrangements dropped in quality considerably when compared to the originals. Most notably Ecruteak and Route 29. Most had redundant and Coda rearrangements that didn't make sense. However, Kanto's remakes sounded somehow just slightly better in the current standard than it did in Gen III, despite using Gen II pieces and arrangements.
(Route 209, Twinleaf, Gateway Colosseum, Kanto Trainer)