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Starters: 4 has the best choices, all are excellent. I do love the starters in gen 1 and 2 as well, but I tend to prefer one individual over the others (Bulbasaur and Totodile).
Region: Johto is the comfiest region, plus you get to visit Kanto as well.
Rival: Blue/Green is the most competent rival, doesn't pester you constantly like rivals in later gens and has a well balanced team.
Music: 1 comes close, but dat Olivine City theme. Music had nice simple melodies without any instrumental clutter.
Story: Kanto has a nice simple story, more about you going on an adventure and going where you want. Minimal hand holding and the story isn't all about stopping an evil team, unlike Unova and Kalos.
Legendaries: They were relevant to the story, yet they weren't mandatory to catch to progress further through the story.
Evil Team: Cyrus was one of the best bosses, and Galactic were definitely the evilest team.
Champ: Cynthia is probably the toughest champ to beat, though I do like Lance and Blue/Green too.
Fossils: Archeops was great and Carracosta was pretty sweet too, plus you can get them pretty early on.
Pseudos: Garchomp is probably objectively the strongest pseudo, but my favourite has to be Dragonite, with Tyranitar in a close second