Everyone probably wants Typhlosion because of these reasons:
1. It's the only good Fire-type you can obtain in the game before it's too late.
2. It's a little better than the other two starters.
3. It's the easiest starter to get going with (first gym kills Chikorita, Sprout Tower kills Totodile, and you can't get any good pokemon other than your starter until you beat the first gym thanks to the idiot who stops you from getting into the area south of Violet City where you find Mareep and Wooper).
They're the most common reasons.
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